I don't really understand the ins and outs of your configuration but ...

TELNET to the host address of the RFC822 address may not be meaningful.
One really and truly needs to take a look at the DNS RRs for MX, CNAME,
A.  This should be done on each of the authoritative DNS RRs NS, as well
as any local resolvers that your LISTSERV's SMTP "client" is likely to
use, both looking for inconsistencies, as well as notating the Internet
hosts involved in delivery.

FURTHERMORE due to the LISTSERV(R) Distribute Algorithm for
single/multiple subscribers (with related subscription options), the
LISTSERV(R) Distribute backbone nodes traversed could be different for
the same set of userids on two different lists.  Thus, you then have to
check each of the LISTSERV hosts mentioned in the distribute path to see
if in fact they are responding e.g., mail them each a THANKS command and
hope they are courteous ;-)

Finally, the use of multiple MX hosts sometimes obfuscates the delivery
mechanism when those lower priority hosts have different (send) mail
configs.  I see this quite often on our active 24x7 list where some
userids will bounce a couple of messages due to "we don't relay" rejects
for a few of them.
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