Internet <-> Router <-> Servers

I have serval Server in my network
& my router can alias the Multi-Public IP to different private-IP
only the iredmail server open port 25 & 110 in router

case:
web server - 1.2.3.4
iredmail server - 1.2.3.5
ftp server - 1.2.3.6

when I send out email to Gmail & look in the source of mail
I can found that, sometime the email send out from ip 1.2.3.4 / 1.2.3.6

Now can I fix the IP it send out