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.]