Some Internet providers and mobile phone companies block outgoing connections on port 25. This is an attempt to prevent spam being sent directly from the connected computers (it works quite well, so is not a bad thing to do).
Because of this, you may want to tell VPOP3 to allow incoming connections on another port (the SMTP Submission port 587 is often used).
VPOP3 Basic can only listen on one port per service, so you will have to choose whether to allow connections on the standard port 25 or the alternate port (eg port 587). If you receive mail using incoming SMTP, then you must listen on port 25 as that is the port that all other mail servers will use to send mail to you. If you change the port, then you must change the SMTP settings in all your email clients.
Note that some firewalls/routers can perform "port mapping", so the can allow incoming connections at port 587 on the firewall to be routed to port 25 on the VPOP3 server. Check with your firewall documentation or network support company to see if this is possible.
To change the SMTP service port, go to Services -> SMTP Server in the VPOP3 settings. Click on the Edit Bindings button in the General tab. Double-click on the port number (defaults to 25) to edit it, and enter the new port number that you want VPOP3 to listen on. Then press Submit to save the change.
VPOP3 Enterprise can listen on as many ports as you wish per service. Also, you can create multiple SMTP services with different configurations if you wish.
To set alternate ports for the default SMTP service, go to Services -> SMTP Server in the VPOP3 settings. Click on the Edit Bindings button in the General tab.
To add a new port, click on the New button, and double-click the port column to edit it. Then enter the new port number (eg port 587). Press Submit to save the change
To remove the new port you've just added, select the relevant row in the table, and press the Delete button, then press Submit to save the change.