Hello,
While I have not tried this with emails, you can try using mwan3 (failover/load-balancing feature) to route packets that match email protocols via mobile interface. This is similar to this case here where the screenshots show an example with DNS (port 53). In your case, this will be ports associated with email protocols. You can find protocols and their port numbers online.
For example, SMTP:
- 25: Non-encrypted
- 465: SSL/TLS
- 587: STARTTLS
POP3:
- 110: Non-encrypted
- 995: SSL/TLS
IMAP:
- 143: Non-encrypted
- 993: SSL/TLS
Keep in mind that email protocols use TCP, and not UDP.
Kind Regards,