Some ISPs will need you to send mail using port 587, instead of port 25.
Port 587 is the 'SMTP Submission' port which is commonly used instead of, or as well as, the standard SMTP port 25 for when users are sending mail to a smart host/relay server.
To tell VPOP3 to use port 587, in the SMTP Relay Servers box, type the SMTP relay server name, followed by “:587”, eg smtp.gmail.com:587
This information is also provided if you hover over the hint '?' symbol next to the entry box.