5 SMTP misconceptions that trip up email senders

SMTP has been around since 1982. You’d think we’d have it all figured out by now. And yet, some of the most persistent email delivery problems trace back to a handful of stubborn misconceptions about what SMTP actually does.

The protocol does one job – transporting email between servers – and it does that job well. But it doesn’t authenticate your identity, guarantee inbox placement, encrypt your content by default, or handle bounces the way most senders assume. Understanding where SMTP stops and everything else starts is the difference between a clean email infrastructure and one that’s always on fire.

SMTP – Simple Mail Transfer Protocol – is a transport protocol. It moves email from one server to another. That’s its entire jobThink of SMTP as the delivery driver who picks up a package and drops it at the loading dock of your recipient’s building. What happens after that – whether anyone’s home, whether the package gets flagged at security, whether it lands on the right desk – is entirely outside the driver’s control.
SMTP governs the handshake between servers: the EHLO greeting, the MAIL FROM and RCPT TO commands, the message transfer, and the final 250 OK response confirming acceptance. Once that 250 lands, SMTP considers its work done. Whether the message reaches the inbox, the spam folder, or nowhere at all is a separate question – and that’s where most of the misconceptions live. If you want a thorough walkthrough of the full SMTP sending process, we’ve covered it in detail.

This is probably the most expensive misunderstanding in email. A 250 OK response from a receiving server means that server accepted your message – not that it delivered it to the inbox. The receiving server is free to do whatever it wants with the message after acceptance: filter it to spam, quarantine it, or silently discard it.

SMTP operates at the transport layer. Inbox placement is a deliverability question, governed by sender reputation, authentication signals, engagement history, and content signals – none of which SMTP touches directly. When you’re debugging low open rates or high spam placement, don’t look at SMTP logs alone. Look at your sender reputation, your authentication setup, and your list health. Our guide to common email deliverability myths is a good place to start untangling the two.

SMTP was designed in a more trusting era. The original protocol has no mechanism to verify that the sender is who they claim to be – it just takes the MAIL FROM address at face value. That’s why email spoofing has been a problem for decades.

Authentication is layered on top of SMTP, not baked into it. SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) all operate independently of the SMTP handshake. SMTP AUTH – the mechanism that requires a username and password before your mail client hands off a message – is also a separate add-on, defined in RFC 4954, not in the core SMTP specification.
What this means in practice: a 250 OK from a receiving server doesn’t confirm your authentication records are in order. You can successfully transmit a message over SMTP and still get junked or rejected because your SPF record is broken or your DKIM signature doesn’t verify. Fix the authentication first – SMTP won’t do it for you. Our email authentication guide walks through SPF, DKIM, and DMARC setup from scratch.

Out of the box, classic SMTP sends everything in plaintext. Email headers, message content, credentials – all of it potentially visible to anyone monitoring the connection. This wasn’t a critical concern in the early internet; it’s a serious problem today.
Encryption came later, as a separate layer. Two options cover most sending scenarios:

STARTTLS
STARTTLS is a protocol extension that lets a client upgrade an existing plaintext SMTP connection to an encrypted one. It’s supported on ports 25 and 587. The key word is “upgrade” – if the receiving server doesn’t support STARTTLS, the connection can fall back to plaintext. This is sometimes called opportunistic TLS, and it’s the dominant approach for server-to-server relay.

SMTPS (implicit TLS)
SMTPS opens a TLS-encrypted connection from the start, with no plaintext phase. It runs on port 465 (for client-to-server submission) and is the more secure option when both sides support it. The practical takeaway: always configure your sending to use TLS, and confirm your ESP enforces it. Our deep dive on SMTP ports covers the security implications of each port choice. Don’t assume that because your message was accepted, the connection was encrypted.

Port 25 is the original SMTP port, dating back to 1982. It’s also the port most likely to get you into trouble.
Most ISPs and hosting providers block outbound port 25 on non-server networks specifically because it’s so heavily abused by spammers. If you’re building an application that sends email and you’re routing through port 25, you’ll hit connection errors on many networks – and your messages won’t leave.
The ports to know:

Port 587: The modern standard for authenticated email submission (SMTP with STARTTLS). Use this for client-to-server submission in almost every case.
Port 465: Deprecated for a period, but now widely used for SMTPS (implicit TLS). Many ESPs, including Mailgun, support it.
Port 2525: A practical fallback when 587 is blocked. Mailgun supports this.
Port 25: Server-to-server relay only. Not for client submission. Often blocked.

For a full historical breakdown and a clear recommendation for your setup, see our SMTP port guide.

This one tends to surface when teams want to reduce their ESP costs or keep email infrastructure in-house. The logic feels reasonable on the surface: how hard can an SMTP server be?

In practice, maintaining your own SMTP infrastructure is one of the fastest ways to degrade your deliverability without realizing why. Running your own server means you’re responsible for everything SMTP doesn’t handle: IP warm-up, bounce and complaint processing, suppression list management, feedback loop registration with major mailbox providers, TLS certificate maintenance, blocklist monitoring, and staying current with evolving sender requirements from Gmail, Yahoo, and Microsoft. That’s a full-time job – and it’s work that ESPs have spent years building tooling around.

Beyond the ops overhead, self-managed servers are often on shared IP blocks with poor reputations, and new IP addresses face steep inbox placement hurdles until a warm-up period establishes a sending history.

For most teams, SMTP relay handles the infrastructure complexity and gives you logs, analytics, and deliverability tooling you’d otherwise have to build yourself. And if you’re weighing whether to stay on SMTP or move to an API integration, our SMTP vs. API comparison lays out the tradeoffs clearly.

SMTP is not the enemy. It’s a remarkably durable protocol that’s been routing email for over 40 years, and understanding what it actually does is the first step to building a clean sending setup.

The short version: SMTP moves your message from point A to point B. Everything else – inbox placement, authentication, encryption, bounce management – is your responsibility to configure on top of it. The good news is that modern email infrastructure, from authentication protocols to managed relay services, makes that layering straightforward if you know where to start.

If you’re troubleshooting a specific send failure, our SMTP error codes guide will help you decode exactly what the receiving server is telling you and how to fix it.

Author: The Sinch Mailgun team The Sinch Mailgun team shares news, best practices, and strategies to take your products and apps to the next level using email. Subscribe to our newsletter to get all the articles in your inbox!