Given the case of multiple domains on one account with one unique IP, what do the outgoing email headers "look" like? Do they reflect the domain sending them, or the hostname of the IP, or something else?
Will each domain use its own name for its mail servers, or will they all use the "dominant" domain name?
Mainly, I'm wondering what effect the choice of the "dominant" domain has on other stuff, given one fixed IP. Clearly, all the other domains will reverse resolve to it. What other issues should I be aware of, when choosing which domain to make the primary?