This is needed for BackupPC which is used in many thousands of RHEL/CentOS servers and admins are starting to move to EL 8.
perl-DateTime-Format-Mail-0.403-6.el8 is available in the Code Ready Builder (RHEL) / PowerTools (CentOS) repository for EL-8. Packages from EPEL8 often include dependencies from there so you need to enable one of those repositories. See: https://fedoraproject.org/wiki/EPEL#How_can_I_use_these_extra_packages.3F
Interesting... From the blocked bug you can see it couldn't be installed and he must already be running EPEL for the other dependnecies. No packager test instance of CentoS 8 is setup yet (last I checked). I guess I'll need to setup a VM to diagnose.
Some EPEL packages will install OK without enabling PowerTools, others won't. He must have enabled EPEL but not PowerTools.