Postfix warnings: hostname XXX does not resolve to address YYY

[v60fan]

I’ve started using MIAB with a box that has IPv6 enabled alongside IPv4. I’m noticing some mail.log entries which I cannot say I’ve seen before (when using IPv4 only), and I have dozens of them today.

The pattern I notice is that there is a connection to the box on IPv6 but the resolution being attempted on IPv4 and not matching, noted like:

postfix/submission/smtpd[25749]: connect from box.domain.tld[IPv6 address]
postfix/smtpd[25749]: warning: hostname box.domain.tld does not resolve to address IPv4

Searching Postfix documentation, I have found:

By default Linux /etc/hosts lookups do not support multiple IP address per hostname. This causes warnings from the Postfix SMTP server that “hostname XXX does not resolve to address YYY”, and is especially a problem with hosts that have both IPv4 and IPv6 addresses. To fix, turn on support for multiple IP addresses:

Therefore I’m wondering do we need the fix it suggests in main.cf, with RESOLV_MULTI=on?

Alternatively, specify the RESOLV_MULTI environment variable in main.cf:

     /etc/postfix/main.cf:
          import_environment = MAIL_CONFIG MAIL_DEBUG MAIL_LOGTAG TZ XAUTHORITY DISPLAY LANG=C RESOLV_MULTI=on

There have been two prior forum posts with similar issues and I’m wondering if they have IPv4 and IPv6 both enabled?
Strange entries in mail.log
Postfix: Hostname not resolving to IP

[zoof]

I noticed this myself in my pflogsum reports and just ignored it. As far as I am aware, everything still functions. Do we need to be concerned?

[v60fan]

Yes, I think everything works. My suggestion on configuration was simply one of “would it be more correct?”. I think it can be ignored, but I am not sure, so I felt best to raise it and gather some discussion.