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SMTP, IMAP, and POP3 Ports: Secure Defaults and MailSlurp Testing

Use this SMTP, IMAP, and POP3 ports guide to choose secure defaults, configure MailSlurp mailbox access, and test send and receive workflows.

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Email port issues usually look like broken mail, but they are often configuration mismatches. SMTP, IMAP, and POP3 all use different ports because they solve different jobs:

  • SMTP submits and relays outbound email.
  • IMAP reads and synchronizes mailbox state on the server.
  • POP3 downloads messages from a mailbox to a client.

Use this guide to choose secure defaults, understand where MailSlurp ports fit, and test the full send-and-receive workflow instead of checking a socket in isolation.

Quick answer

For most modern setups:

  • use SMTP submission on 587 with STARTTLS when the provider supports it
  • use SMTP implicit TLS on 465 when the provider documents that mode
  • use IMAP over TLS on 993 for synchronized mailbox access
  • use POP3 over TLS on 995 only when a download-first workflow is required

MailSlurp also exposes mailbox-compatible SMTP and IMAP access for supported inbox workflows, while the MailSlurp API remains the most deterministic option for automated tests.

MailSlurp SMTP and IMAP ports

The following ports and hosts are available for MailSlurp SMTP inboxes and mailbox-compatible access.

Protocol Encryption Port Host
SMTP None 25 mx.mailslurp.com
SMTP None 2525 mx.mailslurp.com
IMAP None 1143 mailslurp.click
SMTP TLS 587 mailslurp.mx
SMTP TLS 2587 mailslurp.mx
SMTP TLS 465 mailslurp.mx
SMTP TLS 2465 mailslurp.mx

Common email ports

Protocol Port Security model Common use
SMTP 25 server-to-server or restricted relay mail transfer, not normal app submission
SMTP 587 STARTTLS recommended app submission default
SMTP 465 implicit TLS supported by many providers for secure submission
SMTP 2525 provider-specific fallback fallback when 587 is blocked and provider supports it
IMAP 143 plain or STARTTLS legacy or explicit TLS upgrade access
IMAP 993 implicit TLS secure IMAP mailbox access
POP3 110 plain or STARTTLS legacy POP3 access
POP3 995 implicit TLS secure POP3 mailbox access

How to test port configuration

Port checks should answer three questions:

  1. Can the client reach the host and port?
  2. Does the server expect plain, STARTTLS, or implicit TLS?
  3. Does a real message arrive and expose the expected headers and body?

Use SMTP tester for transport diagnostics, then capture the resulting message in Email Sandbox or assert it with Email integration testing. When sender identity matters, inspect the received copy with the Email header analyzer.

SMTP port testing workflow

  1. Confirm whether the provider expects 587 with STARTTLS or 465 with implicit TLS.
  2. Send a controlled test message through the application or SMTP client.
  3. Capture the message in a MailSlurp inbox.
  4. Check From, Return-Path, Received, and Authentication-Results headers.
  5. Assert the subject, body, links, OTP codes, and attachments in CI.

If the SMTP server accepts the message but the inbox never receives it, keep the SMTP transcript, message ID, and received-message search evidence together. That keeps debugging focused on relay, DNS, authentication, content, or recipient policy instead of guessing.

IMAP and POP3 port testing workflow

For IMAP:

  • connect on the expected host and port
  • verify folders, flags, and read state sync across clients
  • confirm the message remains available on the server
  • check the same message through MailSlurp API when test determinism matters

For POP3:

  • connect on the expected host and port
  • verify messages download in the expected order
  • confirm whether the client deletes or leaves messages on the server
  • avoid using POP3 as the main test fixture for parallel CI runs unless the workflow is intentionally download-first

Use IMAP vs POP3 when the team is deciding between synchronized mailbox access and local download behavior.

Common mistakes

Using SMTP settings for IMAP or POP3

SMTP sends mail. IMAP and POP3 retrieve mail. A correct SMTP host does not prove incoming mailbox access is configured correctly.

Mixing STARTTLS and implicit TLS

Port 587 usually starts plain and upgrades with STARTTLS. Port 465 usually expects TLS immediately. Swapping those modes is a common reason a client cannot connect.

Treating port reachability as delivery proof

A successful connection is not the same as a completed email workflow. For release checks, prove the received message, headers, links, and events.