My ISP has recently blocked the port 25, so now I can't send e-mails. I know that the recommended alternative port is 587, but not in case of my mail provider. How to check the correct SMTP port number(s)?
465 is the SSL/TLS port (smtps), versus the usual port 25 (smtp), 587 is an alternate for several 'redirectors', like gmail.com, Dyn.com and others. (These I know). Usually used to get around your ISP's port 25/465 block.
If you use your ISP's given outgoing email server, it'll work just dandy on port 25. They prevent outgoing usage of port 25 to limit spammers and the like.
There are several solutions:
You're trying to find the non-port-25 connection for EVERY outgoing email? Good luck. Many don't have alternate incoming ports, since they don't need them.
Use a properly accepted method to send email, you'll do fine. Using your ISP's given server is the cheapest and best way. Hopefully you're not sending spam. Dislike those people.
Most IPS's have a support page that lists the ports and smtp addresses you connect to in order to check mail.
A common port you will sometimes see used aside from 25 and 587 is 465 although this one is less commonly used because it is not officially mentioned in the RFC.
It's possible that they have a new SMTP server as well if they recently started blocking port 25.
Are you seeing port 25 blocked trying to send through an external SMTP provider (not your ISP server), or are they no longer accepting connections on their own server via port 25.
Often times, you can call them and request they stop blocking port 25 for you if using it as absolutely necessary.