Apologies if this has been asked before, but I need a quick sanity check
from the trenches:

For a variety of reasons, I'd like to move e-mail services in-house from
our current hoster. For now this means port-forwarding to/from a filtering
gateway with the only public address currently available to us.

Our current e-mail hoster allows user editing of DNS records for mail and
webmail (.our-domain.com); however, the gateway's WAN address belongs to a
different provider.

What's the sanest approach to setting up those records? Obviously, there
already exists an A record for the WAN address with the original owner, so
can I just have yet another A record for each service (mail,
webmail) pointing to the same address from within my zone file? Wouldn't a
CNAME setup be the better approach? But, then again, I faintly recall some
issues with MX pointing to CNAMEs.

Any pointers greatly appreciated.