TO AMEND YOUR SMTP PORT IN MACMAIL
Some ISPs (Internet Service Providers) will block sending mail on the default port 25. Follow the Steps Below to take you through changing your port to 587 which will allow mail to be sent.
Start Mac Mail, select Mail » Preferences
Highlight the account you want to change the outgoing port for, select the dropdown beside Outgoing Server (SMTP) then click Edit SMTP Server List
Click Use Custom Port and enter 587 and click OK
Close the Accounts Window
You should now be able to send mail via our outgoing mail server smtp.iomartmail.com without being blocked by your ISP