Sooo… I just happened to stumble over this post here… Katello Error Syncing deb based repo packages, and thought I’d give it a shot.
Updated the emtpy download policy to “immediate”, resync’ed repositories, republished content views, cleared client cache, re-subscribed client system, and lo and behold
$ sudo apt search mailutils-common
Sorting... Done
Full Text Search... Done
mailutils-common/unknown 1:3.7-2.1 all
common files for GNU mailutils
$ sudo apt install mailutils-common
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
mailutils-common
0 upgraded, 1 newly installed, 0 to remove and 1 not upgraded.
Need to get 272 kB of archives.
After this operation, 1615 kB of additional disk space will be used.
...
Success!