Thank you, Rolf

I run Mercury on my home broadband connection and having changed ISP I no longer have a static IP.    I send outgoing mail via a smarthost and MercuryC, no problem.

For receiving mail, I have registered on duckdns.org and have a subdomain there that points to my current dynamic IP. I've set the MX of my domains to this and I receive mail. However, I am a little concerned that I can't set Mercury up to be fully compliant, with a valid PTR record and EHLO. How much does that matter for receiving mail, and is there any recommendation for what I should set the canonicial name and EHLO to in mercury.ini?

Thanks

MercuryS should be set to respond with the domain name or the hostname (like mydomain.com or mail.mydomain.com). Nothing much you can do about the PTR record in this case, but that is usually not a problem. There should be a PTR record, but it isn't required to match your hostname in any way.