I have a working postfix/dovecot/mysql mail server per Linode's most recent guide on Deb. 7, in that at least I can send/receive mails for multiple users on multiple domains.

I've found errors in the mail logs stating, for example "Comcast requires that all mail servers must have a PTR record with a valid Reverse DNS entry."

Consider that the domain is domain.com, the mailserver used for email clients is mail.domain.com, and the FQDN is frank.domain.com

There is a valid rDNS entry for domain.com, but apparently there is no way to enter another rDNS entry for mail.domain.com or frank.domain.com (I'm not sure which would be better, anyway).

Does anyone have a solution for this? If I change myhostname in /etc/postfix/main.cf from frank.domain.com (as currently set) to just domain.com, will that break everything? Or should I set rDNS for the main IP address to the FQDN?

Thank you for any assistance.

PTR's should point to the HOSTNAME (so in your case frank.domain.com)

_________________
Either provide enough details for people to help, or sit back and listen to the crickets chirp.
Security thru obscurity is a myth - and really really annoying.

Basically forward and reverse lookups should match. So if I lookup 1.2.3.4 and get example.com then I should be able to lookup the A record for example.com and get 1.2.3.4. If the two values don't match then you don't have valid rDNS.

(Same is also true for IPv6 and AAAA records).

The name you set inside your MTA _need not_ be the same as the hostname or the rDNS lookup name, but it _should_ also be resolvable. Normally it's recommended to be the same as the rDNS value if you only have 1 IP address, but it doesn't have to be.

_________________
Rgds
Stephen
(Linux user since kernel version 0.11)