Some intermittant delivery (generic) problems are because some sites have
multiple MX hosts which do not handle e-mail identically e.g. "we do not
relay" or "userid not found"  when the highest precedent MX host does
just fine.  Add to that list A/V and anti-spam protection of various servers.

Then further add to that list the vagaries of DNS processing (out of
synch among servers) and on top of that, the perculiarities of the
LISTSERV Distribute Backbone.

/Pete