[Exim] Hostname used in HELO

Hi everyone,

I'm pretty new to Exim, but I've been doing a lot of reading of the Exim
4 specification and am very impressed with what I read.

There's one thing I'm still confused about however, apologies if I've
missed this being covered in any of the available documentation.

If my system is using an IP from an ISP that reverse-resolved to
pc-194-154-100-117.myisp.com, but I refer to my box as
server.mydomain.com, which is a domain under my control with an A record
pointing to my IP address. I have my system's hostname set to
server.mydomain.com, so this is what Exim uses when primary_hostname
isn't set.

The problem is that when Exim delivers mail to remote hosts, some of
them (notably FreeBSD.org) require that you use the same hostname in
your HELO command as your IP reverse-resolves to. This is supposed to
catch MTAs commonly used by spammers.

When I checked out qmail, it had an option to alter the hostname used in
the HELO command, but I couldn't find one for Exim? I'd rather not just
simply set primary_hostname because then Exim uses the ugly .myisp.com
in its bounce messages and other places.

Can anyone help or point me at the right part of the documentation?

Thanks!

Andrew.