Certainly possible, I have 2525 as my SMTP port. Another option is 465 for SMTP over TLS and 587 as already mentioned. Have you checked the home router is not blocking the relevant packets to your Pi?

Check the target port is open with telnet. Through a mobile connection try and talk via telnet to the home system on that port.

Check home ports up (Debian);<br/>
sudo netstat -plant

Check open;<br/>
remote$ telnet home.example.com 25

Other than that just follow the configs.