SMTP port number
When it comes to configure an SMTP server on a mail client it's imperative to set the correct port number. The default port number of an outgoing SMTP server is 25: but a switch to another one can be needed: for instance, if you are facing issues with the SMTP connection (the client is not sending emails even if the server name is right) you can try to change the port as a troubleshooting measure.
As a matter of fact, port number 25 can be blocked by some ISPs because of the ever increasing quantity of spam and malware that transit by there. So a good solution is to simply change the port to 587 or 2525 (if you're using an unencrypted connection) and 465 or 25025 for a SSL connection.
Remember anyway that only a professional SMTP service can guarantee the highest delivery rate, avoiding completely all these sending problems. With turboSMTP you can get immediately 6000 free relays/month, forever: give it a try.