|Mail is sent to a mail server using SMTP (Simple Mail Transfer Protocol) . SMTP normally uses port 25. However that is not written in stone, Some mail servers may use a different port in order to combat spammers. |
One thing to be aware of, you cannot send mail to Outlook Express. Outlook Express is a mail client, not a server, therefore it can only fetch mail from a server using POP3 and port 110 or send mail to a server using the aforementioned SMTP.