As a user, Eudora has email filtering.  I suspect your client does too.

You can't force the recipient to upgrade their software.

You may need to put them (as opposed to they doing themselves) into DIGEST mode.

When I run certain lists, it was my habit of:

SET listname NOMAIL NOPOST FOR bouncer@host

When the individual came back and saw this, IF THEY SAW IT, I would send them boilerplate about their OOOs and my inability to process them repeatedly on a multi-K subscriber list.

Some "got it." 

/Pete


PS -- there were two reasons for our including NOPOST along with NOMAIL ... see if you can come up with good reasons.  We were already using REVIEW for new subscribers too.  [Don't email me your answer.]