Both servers are running SRS already.
I think you getting mixed up here the first machine is a clients pc running outlook, which sends via the mail server SM1. If this was the case every mail client would need a dedicated ip address, the only 2 devices that need static ips and they have them is the mail servers themselves. This issue happens if a mail is sent via cellphone, pc, etc, I cannot believe that this is the NORM, before SM12.4 we never had this issue. howecome all of a sudden,
SM suggests that we set the domain as a trusted sender on the 2 domains, This I feel is nonesence, this means effectively if any domain on the 1st server needs to send to a domain on the other servers, of which we have 6, they have to be setup as trusted senders to allow mails to be sent between them.