Are you finding that your emails still end up in the spam folder? For sales outreach and marketing teams, this is a constant frustration. While many factors can influence inbox placement, one often overlooked element is the SMTP port (Simple Mail Transfer Protocol port) your email service provider is using.
SMTP is the protocol that transmits your messages, and the port you choose plays a key role in whether emails are delivered securely or flagged as spam.
In this guide, we’ll explain what SMTP ports are, why they matter for deliverability, and compare the most common options so you can decide which SMTP port best fits your email program.
TL;DR: The SMTP port you use directly impacts email deliverability and spam placement. Port 587 is the recommended standard for secure, inbox-friendly email sending, while outdated ports like 25 or 465 can increase spam risks and blocking by ISPs. Port 2525 offers a reliable fallback, especially for cloud-based senders. If emails still land in spam, factors beyond ports — like domain reputation and list health — may be to blame. Allegrow helps uncover those hidden issues, providing real-time monitoring, sender reputation insights, and deliverability protection.
What is SMTP?
SMTP, or Simple Mail Transfer Protocol, is the standard used to send email across the internet. When you send a message, SMTP transmits it from your email service provider (like Outlook or Gmail) to the recipient’s mail server, making sure it’s routed and delivered correctly.
Over time, SMTP has evolved to support the growing needs of the internet. Today, it’s still the backbone of email communication and a critical piece of any marketing or outreach program.
Where teams often get stuck is in the configuration. Choosing the right SMTP port for submission is essential — it determines how securely your emails travel and whether mailbox providers treat them as safe or suspicious. In the next section, we’ll explain what ports are and why they matter for deliverability.
What Are SMTP Ports and Why Do They Matter?
An SMTP port is the channel (or “doorway”) your email uses to travel from your system to another server. Each port is tied to a specific protocol or service, and some ports are reserved for handling SMTP traffic
Not all ports behave the same way. Certain ports are built for secure, encrypted delivery, while others are outdated or frequently blocked by internet service providers (ISPs) to stop spam. If you choose the wrong port, emails may bounce, get delayed, or land in the spam folder — undermining your sales and marketing efforts.
It’s also important to understand “default ports.” These are well-known ports widely supported by mail servers, but they are also common targets for abuse. Knowing which SMTP ports to rely on — and which to avoid — is key to improving email deliverability and keeping campaigns running smoothly.
Which SMTP Port Should You Use? A Look at the Most Common Options
Port 25 — The Original SMTP Port (But Often Blocked)
Port 25 is the original and most widely recognized port for SMTP. Historically, it has been the default port for sending emails between servers. However, due to its age and widespread use, Port 25 has become a target for spammers. As a result, many ISPs now block this port for outgoing mail to prevent spam from being sent over their networks.
Despite its limitations, Port 25 is still used for server-to-server communication.
If your emails are consistently being marked as spam, it could be because you're using Port 25, and switching to a different port may help improve deliverability.
Port 587 — The Best SMTP Port for Sending Email
Port 587 is the most up-to-date and recommended port for sending emails. Known as the submission port, it’s designed to handle email transmissions from clients (like email marketing tools) to mail servers. Unlike Port 25, Port 587 supports encryption, which ensures that your emails are transmitted securely.
Using Port 587 offers several advantages, including better compatibility with modern email systems and improved security. It’s considered the best practice for sending emails, especially for businesses and marketing professionals who want to ensure their messages reach their audience safely.
Port 465 — The Deprecated SSL Port You Should Avoid
Port 465 was initially designated as the port for SMTP traffic secured by SSL (Secure Sockets Layer). However, it was later deprecated in favor of Port 587, which offers more flexibility with encryption protocols. Despite this, some email service providers still support Port 465 for legacy reasons.
While it’s not recommended to use Port 465 for new email setups, if you’re working with an older system that requires it, make sure to check whether your email provider still supports SSL encryption on this port. If not, you should consider upgrading to a more secure option like Port 587.
Port 2525 — A Flexible Alternative SMTP Port
Port 2525 is an alternative SMTP port that’s gaining popularity, especially among cloud-based email providers. It’s not a standard port in the official sense, but it’s supported by many email services as an alternative to Port 587. One of the benefits of using Port 2525 is that it’s less likely to be blocked by ISPs, making it a good fallback option if you encounter issues with the other ports.
How to Choose the Right SMTP Server for Your Email Program
Choosing the right SMTP server isn’t only about picking a port — it’s about aligning the service with your sending needs. The first factor to consider is volume. If you send a handful of transactional emails per day, almost any provider that supports Port 587 will work. But if you’re running thousands of marketing or outreach emails daily, you need an SMTP service that can handle high throughput without throttling, while maintaining strong deliverability.
The type of email you send also matters. Transactional emails (like password resets or receipts) usually demand high reliability and fast delivery. Marketing or sales emails, on the other hand, are more sensitive to spam filtering, meaning your SMTP provider must offer strong authentication support and proven inbox placement.
For small businesses or personal projects, your existing email provider’s SMTP service may be enough. For larger organizations or teams running structured campaigns, it’s worth investing in a dedicated or specialized SMTP service. Look for features like:
- Support for modern submission ports (587, 465, and optionally 2525).
- TLS encryption by default.
- Authentication protocols (SPF, DKIM, DMARC) enforced and well-documented.
- Clear sending limits or dedicated IP options for scaling safely.
- APIs or integrations that fit into your existing sales or marketing stack.
Some enterprise-grade providers — such as Amazon SES or other cloud-based SMTP services — may require technical setup or developer involvement. That extra effort often pays off with more flexibility, higher reliability, and stronger control over deliverability settings.
SMTP Port Security: What You Need to Know
Security is a critical aspect of email communication. Ensuring that your emails are encrypted is essential to protect sensitive information and maintain the trust of your recipients. Ports like 587 and 2525 support TLS (Transport Layer Security) encryption, which is now a de-facto requirement for sending emails. Without this layer, mailbox providers will often downgrade your reputation. You can monitor encryption details using tools such as Google Postmaster Tools, which provide visibility into how inbound and outbound traffic is being secured.
Using outdated ports like 25 or 465 without encryption can leave your emails exposed to interception or hijacking, which risks both data loss and brand reputation. Always make security the priority when configuring SMTP settings.
STARTTLS vs implicit TLS (Port 465): handshake differences
Email encryption can happen in two different ways depending on the port. With Port 587, the connection usually starts in plain text and then upgrades to encryption using the STARTTLS command. This approach gives flexibility, since servers that don’t support encryption can still accept mail (though that’s rare today).
With Port 465, encryption is implicit: the TLS tunnel is established from the very first packet, and the entire session is encrypted end-to-end. This “wrapped TLS” method removes the negotiation step and ensures that no part of the conversation is ever unprotected.
In practice, both methods are secure when configured properly. Port 587 is the recommended standard because of its broad compatibility and official role as the submission port. Port 465, while not as universal, remains widely supported by major providers and can be a secure alternative if your infrastructure or ESP prefers it.
Ports don’t replace DMARC, SPF, DKIM, or MTA-STS
It’s important to remember that SMTP port choice alone won’t keep your emails out of spam. Ports control how your message travels, but deliverability depends on authentication and domain-level trust signals. Standards like SPF, DKIM, and DMARC verify your identity, while MTA-STS enforces secure transport policies across domains.
Mailbox providers such as Gmail and Yahoo now require these measures for bulk senders, and failing them can cause messages to be rejected or flagged — even if you’re using the “right” port.
Think of ports as the highway and authentication as your license plate: you need both the road and the credentials to reach your destination safely. For a step-by-step framework on improving inbox placement, see our Email Deliverability Best Practices guide.
Troubleshooting SMTP Port Errors and Connection Issues
If your emails aren’t being delivered or keep landing in spam, your SMTP port configuration may be part of the problem. Here are common issues and quick fixes:
- Blocked Port: Many ISPs and corporate networks block Port 25 to stop abuse. Even if it works today, it can be throttled or blocked without notice. If you’re submitting mail from a client or a sales/marketing platform, switch to Port 587 first. If your network still blocks it, try Port 465 (implicit TLS) or Port 2525 (a common fallback supported by many providers).
- Incorrect Configuration: Two small settings cause most headaches: picking the wrong port and forgetting to enable encryption (TLS/STARTTLS). In most cases, set your client to Port 587 + STARTTLS. If your provider prefers implicit TLS, use Port 465 with SSL/TLS “on from the start.” Save and test after each change.
- ISP Restrictions: Some providers only accept mail on specific ports, require authentication before sending, or limit how fast you can send. Look for a “SMTP settings” or “Allowed ports” page from your provider and match their exact recommendations. If you’re on corporate Wi-Fi, ask IT whether outbound SMTP is restricted.
Once you’ve ruled out these basics, verify each port directly.
Quick port tests (openssl, swaks, telnet)
These light checks confirm reachability and encryption. Replace smtp.yourdomain.com with your host.
- 587 — submission with STARTTLS
openssl s_client -starttls smtp -connect smtp.yourdomain.com:587 -servername smtp.yourdomain.com
(Alt) swaks --server smtp.yourdomain.com --port 587 --tls -q HELO
Expect: A 220 banner, then a TLS handshake (you’ll see TLSv1.2 or TLSv1.3). If it says “must issue STARTTLS first,” enable TLS in your client. - 465 — submission with implicit TLS
openssl s_client -connect smtp.yourdomain.com:465 -servername smtp.yourdomain.com
(Alt) swaks --server smtp.yourdomain.com --port 465 --tls-on-connect -q HELO
Expect: The session is encrypted from the first packet. If you get a certificate warning, verify the hostname matches the certificate. - 25 — reachability only (not for client sending)
telnet smtp.yourdomain.com 25 (or) nc -vz smtp.yourdomain.com 25
(Windows) Test-NetConnection -ComputerName smtp.yourdomain.com -Port 25
Expect: A banner if the server is listening. Many networks block 25; if it times out or refuses, that’s normal on end-user networks.
Helpful tips: Test from a second network (e.g., phone hotspot) to rule out local firewalls. Add -servername to openssl s_client so the certificate check uses the right hostname. If you see a banner but TLS fails, note the exact error wording — it often points to a simple setting.
Detecting blocks vs auth/timeouts
Ran the checks? Here’s how to read the results and decide what to try next.
Use the symptom to decide your next move:
No banner at all (refused, reset, or timeout)
- What it means: Likely a network block or the server doesn’t listen on that port.
- What to do: Try ports in this order 587 → 465 → 2525. Test from a different network. If it works elsewhere, ask IT/ISP to allow the port or follow your provider’s allowed-ports guidance.
Banner appears, but you get errors after connecting (4xx/5xx codes)
- What it means: The port works, but there’s an SMTP/TLS/auth problem.
- Common clues & fixes:
- “Must issue STARTTLS first” → enable STARTTLS (or try 465).
- “Authentication required/invalid” → use the full email as username, verify password/app password/OAuth, and ensure auth is enabled.
- “Too many messages/try again later” → slow down, add backoff, and check provider rate limits.
- “Policy/reputation denied” → even correct ports can be throttled if list quality or sender reputation is poor — address list hygiene and recent bounce/complaint spikes.
It works once, then fails under load
- What it means: You may be hitting per-IP or per-user rate limits.
- What to do: Lower bursts, stagger sends, or request higher limits. If possible, use a dedicated sending IP with warming and proper authentication.
Conclusions & Next Step
Choosing the right SMTP port is crucial for successful email communication. Whether you're sending marketing emails, transactional messages, or personal correspondence, the port you use can affect both deliverability and security.
Here is a quick round-up of the different ports and how to move forward:
- Port 587 is the recommended port for email marketing due to its support for encryption and broad compatibility with modern email systems
- While you can use Port 25, it's generally not recommended due to its limitations and the fact that many ISPs block it to prevent spam
- Port 465 is deprecated and no longer recommended for new email setups
- Port 2525 is a great alternative to port 587, and is less likely to be blocked by ISPs
Overall, Port 587 offers better security and compatibility and is your best bet for email outreach.
If you’ve adjusted your SMPT port settings and you are still struggling with email deliverability and getting your emails out of the spam folder, there may be other factors affecting your email strategy.
Often, the most critical "other factor" is your list quality. Even with perfect port settings, sending to spam traps or invalid addresses will land you in the junk folder.
Discover the true source of your deliverability issues with a 14-day free trial of Allegrow. Use our advanced verification on up to 1000 contacts to find the hidden risks that technical settings can't fix. It's the next logical step to improving your sender reputation and inbox placement.
FAQs on SMTP Ports
Is port 465 still deprecated, or is it now valid alongside 587?
It’s valid again. RFC 8314 explicitly recommends two secure submission options: port 587 with STARTTLS and port 465 with implicit TLS (“SMTPS”). Most senders default to 587 for broad compatibility; 465 is fine when your stack prefers implicit TLS or 587 is blocked. The choice doesn’t change standards compliance so long as TLS and auth are correct.
Does choosing 587 vs 465 affect spam filtering, or do providers care more about authentication and complaints?
Providers care about authentication and recipient feedback — not your submission port. Gmail/Yahoo require SPF and DKIM, a published DMARC policy, one-click unsubscribe for bulk mail, and complaint rates kept under about 0.3%; they do not mandate a specific port. Meeting these rules and keeping complaints low matters far more to placement than 587 vs 465.
When should I use port 2525, and is it secure as a fallback?
Port 2525 isn’t an IETF-assigned SMTP submission port, but many ESPs accept it as a firewall-friendly alternative when 587 is blocked. It supports TLS (STARTTLS) just like 587, so it’s secure when configured properly. Use it only as a fallback; prefer 587 (or 465 for implicit TLS) when available.
Why is port 25 often blocked for outbound mail, and when is it appropriate?
ISPs and cloud platforms commonly block outbound 25 to curb direct-to-MX spam; Microsoft Azure, for example, blocks it by default. Port 25 remains appropriate for server-to-server relay between MTAs, but end-user apps and marketing tools should submit mail via 587 (STARTTLS) or 465 (implicit TLS) through an authenticated relay.
Do Gmail’s 2024+ bulk-sender rules mandate a specific port?
No. The rules focus on authentication (SPF, DKIM, DMARC), one-click unsubscribe, and keeping user-reported spam under 0.3%; they don’t require a particular port. Use 587 or 465 as your environment allows, and prioritize these compliance and reputation thresholds to protect inbox placement.
