Warning: hostname mail.<domain> does not resolve to address <ip>

The Trellis based web server uses a mailcow mail server.
On the mail server log records pile up that always look like this:

warning: hostname mail.<domain of mailserver> does not resolve to address <ip>

Why does the mail server check the IP of the web server against the MX address of the mail server (which it is itself)?
Is there a mail misconfiguration on the web server part or on the mail server part - or both?
I would like to get rid of these log warnings.

Looks like it’s coming from Postfix or smtpd if that helps with Googling. Example: https://www.digitalocean.com/community/questions/postfix-smtpd-hostname-does-not-resolve-to-ip-but-it-does

What seems strange is that the IP address that doesn’t resolve to the mail server domain (mail.example.com) indeed doesn’t belong to the mail server - but to the web server that - apparently sends system/cron emails via the mail server. Could it be that the web server (trellis) is misconfigured so it authenticates with the mail server domain, hence the mail server will check the IP address of the web server against the mail server domain (and fails)?

Edit: Aha! The web server was once used also for mail (now a separate mail server is used), it still got a forgotten reverse DNS record as mail server.

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