Sendmail, DNS and multiple local domains

[Luis Toro]

I have configured a relay host (DS) in all of my UNIX servers to route non-local email. In resolv.conf I have
our official company domain. It seems that we have multiple local domains and what is happening is that the UNIX servers are sending local emails outside via the relay host, to come back in. My Exchange folks are telling me to let DNS determine whats local and whats not, and let it route to the proper relay.
I'm not familiar with how DNS works with MX records, or local domains, and as far as sendmail goes, if it does whats it supposed to do, I don't touch it. Any info would be appreciated.

[Berlene Herren]

When sendmail tries to deliver mail to a domain, it first searches for MX RRs for that domain's name. Once it has a list of MX RRs, it sorts them by priority, lowest numbers first. It then scans this list looking for it's own name; if it finds that, then it deletes the MX RR and all other with equal or lower MX priority. It can never see itself on an MX list, for mail would just loop. sendmail then tries to deliver the mail using the lowest numbered MX RR. If sendmail gets an SMTP connection from this list, it tries to deliver the mail. So your exchange admins are telling you to let sendmail decide which MX record to use. Does this make sense?

Berlene