Assign a specific static ip address for email server

Apologies if this has already been covered.

Sophos XG Firewall (Home Edition) SFVH (SFOS 19.5.1 MR-1-Build278)

I have 5 public static IP addresses provided by my ISP.

I have a single gateway address of xxx.xxx.xxx.9

I have a single WAN port which has all 5 static addresses listed on it. xxx.xxx.xxx.10 is the primary address and 11 - 14 are aliases.

I am looking at an email server for my home project, which requires mail to send and receive to a specific public IP address of xxx.xxx.xxx.12

Currently, all my traffic goes out on xxx.xxx.xxx.10, including the mail from the email server.

I have got a rule in place to receive which works fine.

I have tried different rules, but I can't seem to force the mail protocols from the email server, to go out on 12 instead of 10.

This will probably turn out to be something really simple, but could someone please advise if there is a way to do this?

Thanks in advance,

Barry.