Many Internet service providers hotels and other internet access locations are blocking the common port for sending email. Apple has provided instructions for how to switch the port Apple Mail uses for .Mac/MobileMe but it is equally useful for email accounts with other services.
If one of our consultants has recommended switching your sending port for 25 to 587 follow the instructions at the link below and enter port number 587 when available. In many cases port 465 is another option if 587 appears blocked as well.