Some (all) servers send cron mails with invalid hostnames in the From: envelope field.
Example:
Jan 10 09:42:01 somerset postfix/qmgr[440]: 7C81020D40: from=<postgres@somerset.internal.softwareheritage.org>, size=774, nrcpt=1 (queue active)
Somerset.internal.softwareheritage.org doesn't exist or exists only locally. Random Internet hosts see this:
$ host somerset.internal.softwareheritage.org Host somerset.internal.softwareheritage.org not found: 3(NXDOMAIN)