Please note that ports 25 and 465 are no longer supported, so port 587 must now be used. The original information is included below for reference only.
It’s possible that your ISP (Internet Service Provider) is blocking port 25. Providers like AOL block this to prevent outside users from relaying bulk email off their servers. You should contact your provider to see if port 25 is blocked.
Otherwise you may have incorrectly entered details in your email software for the outgoing server (SMTP) for example, make sure you are authenticating the outgoing mail server as described in other support articles, or software on your device like anti-virus, firewall, Norton, Symantec, etc., is conflicting.
If port 25 is blocked, you could try using our alternative port 587 instead.