If your business emails keep bouncing, timing out, or getting stuck at “connecting to server,” the SMTP port your email client is using could be the reason.
Simple Mail Transfer Protocol (SMTP) ports 25, 465, and 587 each play a different role in how your outgoing mail leaves your server and reaches your recipient’s inbox.
Getting the wrong one doesn’t just cause failed sends — it can mean your emails are travelling unencrypted across the internet.
This guide breaks down what each SMTP port actually does, which one you should use in 2026, and how to fix the most common configuration errors — whether you’re setting up Outlook, a website contact form, or a dedicated Business Email account.
What Is SMTP and Why Do Ports Matter?
SMTP is the protocol, or set of formatting rules, that moves email across the internet from server to server until it reaches its destination, according to Cloudflare’s Learning Center.
A port, in networking terms, works like a mail slot in a large building — data addressed to the building alone won’t reach the right recipient without a specific slot number attached.
SMTP is designed to communicate over a specific port, and depending on the era and the security requirements in use, that has been port 25, 465, 587, or occasionally 2525.
Port 25 — The Original SMTP Port
Port 25 was the original port assigned to SMTP, and it’s still used today for relaying mail between mail servers.
It is not, however, the port you should use to send mail from an email client, website, or application.
Because port 25 typically carries no authentication or encryption by default, it’s a magnet for spam and abuse — which is exactly why many ISPs and hosting providers block outbound traffic on this port for regular customer accounts.
If your application is configured to send on port 25 and mail simply never arrives, a blocked port — not a code error — is very often the cause.
Port 587 — The Widely Supported Submission Port
In 2011, RFC 6409 formally designated port 587 for mail submission, paired with STARTTLS encryption.
STARTTLS works by starting the connection in plain text, then upgrading it to an encrypted TLS connection before any credentials or message content are transmitted.
Port 587 requires SMTP authentication — a valid username and password — before the server will relay your message, which is what makes it dramatically safer than port 25 for submission.
Most email clients, CRM platforms, transactional email tools, and WordPress SMTP plugins default to port 587, and it remains extremely widely supported in 2026.
Port 465 — The Current RFC-Recommended Default
Port 465 was originally used informally in the 1990s for SMTP over SSL, without official standardisation.
That changed in 2018, when RFC 8314 formally recommended implementing SMTPS using Implicit TLS on port 465 as the preferred default for encrypted email submission, according to Cloudflare.
The key technical difference is timing: Implicit TLS on port 465 encrypts the connection from the very first handshake, while STARTTLS on port 587 starts unencrypted and upgrades shortly after.
In practice, both are considered secure when properly configured — but if your mail provider explicitly recommends one over the other, follow their guidance, since exact implementations vary.
Port Comparison at a Glance
| Port | Encryption | Auth Required | Typical Use | Recommended for Submission? |
| 25 | None by default | No | Server-to-server relay | No |
| 587 | STARTTLS | Yes | Client & app mail submission | Yes |
| 465 | Implicit TLS | Yes | Encrypted mail submission | Yes (RFC 8314 default) |
| 2525 | Varies by provider | Yes | Fallback when 25/465/587 are blocked | Only as a fallback |
How to Set or Change Your SMTP Port
In most desktop email clients such as Outlook or Thunderbird, go to your account’s outgoing (SMTP) server settings and update the port number and encryption method to match your provider’s instructions.
For WordPress sites, an SMTP plugin such as WP Mail SMTP or FluentSMTP lets you enter the host, port, and encryption type directly, bypassing PHP’s unreliable default mail function.
For custom applications, update the SMTP configuration in your code or environment variables — most mail libraries expect a host, a port number, and a flag for whether TLS or SSL should be used.
Common SMTP Errors and How to Fix Them
- Connection timed out — usually means the port is blocked by a firewall or your ISP; try the alternative port your provider supports.
- 535 authentication failed — check your username and password, and confirm whether your provider requires an app-specific password.
- Relay access denied — the server isn’t configured to accept mail from your IP or domain; SMTP authentication needs to be enabled correctly.
- Emails landing in spam despite sending successfully — this is usually an SPF, DKIM, or DMARC record issue, not a port problem, and needs a separate fix.
Why Your Hosting Setup Affects Email Deliverability
Many basic shared hosting environments restrict or throttle outbound SMTP traffic to prevent one customer’s compromised account from damaging the shared IP reputation for everyone else.
A dedicated business email solution avoids this shared-reputation risk entirely, since your sending infrastructure isn’t tied to hundreds of other unrelated accounts.
If you’re setting up business email for the first time or migrating away from a generic inbox, Exabytes Business Email is built specifically for reliable, authenticated outbound delivery.
Frequently Asked Questions
Which SMTP port should I use in 2026?
Port 465 with Implicit TLS is the current RFC-recommended default, but port 587 with STARTTLS remains extremely common and fully supported — either is fine as long as authentication and encryption are enabled.
Why is port 25 blocked on my server or by my ISP?
It’s blocked to prevent spam and abuse from compromised machines; it was never intended for submitting mail from a regular client or application in the first place.
Is port 587 or 465 more secure?
Both encrypt your connection when configured correctly — 465 encrypts from the start via Implicit TLS, while 587 upgrades the connection via STARTTLS shortly after connecting.
Can I send email without SMTP authentication?
Not safely. Unauthenticated relays are a major spam and abuse risk, which is why virtually all modern SMTP submission requires a valid username and password.
What SMTP settings does Exabytes Business Email use?
Exact host and port values depend on your specific plan — check your Business Email welcome documentation or the Exabytes knowledge base for your account’s configuration details.
Conclusion
For most businesses in 2026, the safe default is simple: use port 587 or 465 with authentication and TLS enabled, and avoid port 25 for anything other than server-to-server relay.
If email deliverability issues keep coming back regardless of the port you use, the underlying cause is often the hosting environment rather than the port itself.
A properly configured Business Email account removes the guesswork, giving you authenticated SMTP access without fighting shared-IP restrictions.
Sources
Cloudflare Learning Center: What SMTP port should be used? Port 25, 587, or 465?
Mailgun: Which SMTP Port to Use? Understanding ports 25, 465, & 587
