SMTP Ports 25, 465, 587, and 2525
Choose the right SMTP port for transactional email, including port 25, 465, 587, and the common 2525 fallback.
TL;DR
Use port 587 with STARTTLS for most application SMTP relay integrations. Use 465 only when a client requires implicit TLS. Port 25 is for server-to-server mail transfer, not normal app submission. Port 2525 is a common provider fallback, not an IETF-standard SMTP submission port.
What you will learn
- Choose the right SMTP port for app and framework integrations
- Understand the difference between STARTTLS and implicit TLS
- Avoid blocked or misconfigured SMTP submission traffic
Quick port map
| Port | Common use | TLS mode | Use for Postscale app submission? |
|---|---|---|---|
| 25 | Mail transfer between MTAs | Opportunistic STARTTLS | No |
| 465 | Message submission over implicit TLS | TLS from connection start | Only when your client requires it |
| 587 | Message submission | STARTTLS after connection | Yes, preferred |
| 2525 | Provider fallback | Usually STARTTLS | Only if 587/465 are blocked |
For most product teams, the answer is simple: use smtp.postscale.io on port 587 with STARTTLS.
Port 25 is not the normal app submission port
Port 25 is the classic SMTP transfer port used by mail servers to exchange messages. Many cloud hosts and office networks block outbound port 25 to reduce abuse. Even when it is open, application traffic should usually submit to a provider on a submission port instead.
Use port 25 only when you are operating an MTA and understand queueing, DNS, reverse DNS, abuse controls, and retry behavior.
Port 587 is the default for applications
Port 587 is for message submission. The client connects in plaintext, says EHLO, sees STARTTLS, upgrades the connection, then authenticates and submits mail.
Use this shape for:
- Framework mailers.
- CMS plugins.
- Commerce platforms.
- Monitoring tools.
- Legacy products that cannot call a REST email API.
If the client has separate fields for encryption, choose STARTTLS, not SSL, when using 587.
Port 465 is implicit TLS
Port 465 starts TLS immediately, before SMTP commands. Some older clients call this SSL, even though modern TLS is used.
Use port 465 when a client cannot do STARTTLS correctly or its configuration only supports implicit TLS. Do not configure STARTTLS on 465.
Port 2525 is a fallback
Many email providers support 2525 because hosting providers sometimes block 25, 465, or 587. Treat it as an operational fallback, not a standard you should prefer.
If 587 fails, confirm:
- The network allows outbound SMTP submission.
- The firewall allows the destination host and port.
- The client is using STARTTLS.
- The username and password are SMTP credentials, not a dashboard password.
- The visible From domain is verified.
Recommended Postscale settings
| Setting | Value |
|---|---|
| Host | smtp.postscale.io |
| Port | 587 |
| Security | STARTTLS |
| Username | SMTP credential username |
| Password | SMTP credential secret |
After the SMTP server accepts the message, delivery can still bounce later. Use transactional email bounces and webhook retries and idempotency to track final state.
References
Frequently asked questions
- Which SMTP port should I use for transactional email?
- Use port 587 with STARTTLS unless your mail client only supports implicit TLS on port 465.
- Is port 2525 a standard SMTP port?
- No. Port 2525 is a common provider fallback when networks block 25, 465, or 587, but it is not the IETF-standard submission port.
Related guides
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