Exim : change hostname in SMTP greeting

[Shaun <debian@gmsl.co.uk>]

Hi all,

I ran my domain through a DNS health checker type site, just to make
sure I fix any issues. The only 'problem' was that it said the SMTP
greeting on my MXs doesn't match that listed as the FQDN for the domain.
That's because I tend to run many services on the same box so
technically it has many hostnames.

Now if I want mail.mydomain.com in the DNS for the MX record but it's
hostname (hostname -f) is foobar.mydomain.com then if I don't want to
change it's hostname can I not set something in Exim?  I tried setting

MAIN_HARDCODE_PRIMARY_HOSTNAME='mail.mydomain.com' in
update-exim4.conf.conf after some Googling. I did a restart and exim4
complained with :

Stopping MTA for restart:undocumented line
MAIN_HARDCODE_PRIMARY_HOSTNAME='mail.mydomain.com' found in
/etc/exim4/update-exim4.conf.conf, generating exim macro
 exim4_listener.
Restarting MTA: exim4.

But then, strangely, it seems to work if I do an nc localhost 25.

So question: Is this the right (Debian) way of doing it?

Just want my domain to pass all the checks for a correct configuration :)