> Downgrading from postfix-3.4 fails with:
As documented in RELEASE_NOTES.
Incompatible changes with snapshot 20190126-nonprod
This introduces a new master.cf service type 'unix-dgram' that is
used by the new postlogd(8) daemon. This type is not supported by
older Postfix versions. Before backing out to an Postfix 3.3 or
earlier, edit the master.cf file and remove the postlog entry.