Email Not Sending: SMTP Ports, Port 25 Blocking and Practical Fixes
If email can be received but not sent, the issue is often SMTP configuration. Port 25 blocking, 587/465, TLS/SSL and DNS records should be checked together.
One of the most common email problems is simple to describe: messages can be received, but not sent. This often appears in website forms, proposal notifications or email clients.
The first thing to check is SMTP configuration. Incoming mail and outgoing mail are separate concerns. A mailbox can receive messages even when SMTP sending is misconfigured.
Port 25 has historically been used for mail transfer, but many providers block it to reduce spam. For authenticated sending, modern configurations usually use port 587 with TLS or port 465 with SSL.
The difference matters. Port 587 commonly starts as a normal connection and upgrades to TLS. Port 465 starts with SSL immediately. The right choice depends on the mail provider’s documentation.
DNS records also affect delivery. SPF, DKIM and DMARC help receiving servers decide whether your mail is legitimate. Without them, messages may be rejected or sent to spam even if SMTP works.
Application logs are useful. Errors such as authentication failed, connection refused, timeout or certificate verification failed point to different causes. Guessing ports without reading the error wastes time.
A practical order is: confirm SMTP credentials, test 587/TLS or 465/SSL, check DNS records, read the application log and send a controlled test message.