Port 25 Blocking
Many local and national ISPs now block outgoing mail via third party smtp servers (port 25 blocking). ISPs block port 25 to prevent spammers from setting up mail servers on their networks. Unfortunately this also prevents legitimate users from sending mail through third party mail servers. To test if your local ISP is blocking port 25, open a command prompt and type in the following command.
telnet mail.examplestandardsite.com 25

s
If you can telnet to your mail server on port 25 without error, then they are NOT blocking port 25. If you can NOT telnet to your mail server on port 25 without error, then they ARE blocking port 25. If your local ISP is blocking port 25, you will need to change the outgoing mail server setting for all accounts to match the outgoing mail server setting for the email address supplied by your ISP. This will route all outgoing mail through your ISPs mail server. Using your ISPs mail server to send mail will not affect your email in anyway, it will just route it through your ISP's mail servers. It will not affect the "to:" field, the "From:" field, or the "reply to:" address.