Email Notifications Fail Until SMTP Port Reset

Hi,

We have recently set up email notifications, and are having some issues. 

When adding the SMTP config, the test emails are received with no issue. However when a build is run there will be no email sent, and the notification logs will contain an entry like so

 error: com.sun.mail.util.MailConnectException: Couldn't connect to host, port: mail.****.com, 25; timeout 60000;

The server logs won't have an entry for the error.

We have found that setting the port to 0, saving, then setting the port back to 25 will then cause the build notifications to start sending out.

Any ideas why resetting the port to the same value would cause the emails to start sending?