I have a reverse lookup problem that is causing our users some serious problems. We have a correctly configured PTR for our mailserver (217.158.60.30 - mailhost.websights.co.uk)
OK.
We run IMAIL with multiple virtual hosts and most mail is delivered no problem. However, when sending to Supanet we get a "550 EHLO argument does not match calling host"
That's not a reverse DNS issue. There is no problem sending an EHLO/EHLO, just so long as it has an A (or MX) record pointing back to the IP of the IMail server.
Should Imail be presenting itself as mail.catpad.co.uk (which has an IP of 217.158.60.30 - same as mailhost.websights.co.uk) or catpad.co.uk (as it is below)?
You forgot the most important piece of information -- that catpad.co.uk isn't the IMail server (217.158.60.30); it's 217.158.60.40.
So if your IMail server identifies itself as catpad.co.uk, it's technically wrong. Instead, you should use mail.catpad.co.uk as the official name, with catpad.co.uk as an alias. This does, however, have the unfortunate side-effect of forcing you to forever accept E-mail addressed to @mail.catpad.co.uk. But there isn't anything that you can do about that, AFAIK.
If I change the configuration of the domain in Imail so that it's official host name is mail.catpad.co.uk instead of catpad.co.uk it works, and supanet accepts the mail.
Correct.
As I understand it when Imail connects to a remote server, it can do a reverse lookup on the IP address and as long as there is an entry (mailhost.websights.co.uk in our case) all is well
In most cases, that's true. But some mailservers verify that the hostname resolves back to the same IP. The RFCs say you aren't allowed to block mail as a result of that problem, but some people do it anyway.
Does the helo have to be the mx record for the domain?
The HELO/EHLO can be anything -- just so long as it resolves to the IP of the server.
If supanet looks up the IP for catpad.co.uk it will be the web server IP not the mail server IP.
And that's exactly what they are doing, and exactly why they don't accept your mail.
-Scott
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