TO AMEND YOUR SMTP PORT IN MOZILLA THUNDERBIRD
Thunderbird is a free application any customers using an older version than 17, we suggest you upgrade direct from Mozilla - http://www.mozilla.org/en-US/thunderbird/ as the info below is specific to Thunderbird 17 or higher
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 Mozilla Thunderbird, select Applications Menu » Options » Account Settings
Select Outgoing Server (SMTP) then highlight the account you want to change the outgoing server for and click Edit.
Change the Port to 587 and click OK
You should now be able to send mail via our outgoing mail server smtp.iomartmail.com without being blocked by your ISP