Russ:

"I've done that. I'm not convinced they see it as their problem, or at least that they did the first couple of times it came up. "


Yeah.  No.  If the site is blocking mail from "[log in to unmask]" that is a list owner problem.  Maybe something in any template used or whatever …

But if it is blocking your host "listserv.domain" then the site administrator may have a problem.  You can check:
1.  set up another list … no special configuration say "RUSS-TEST").
2.  You'll need to work with your site admin and the sysadmin on the receiving site, and maybe some of the (former) subscribers.
3.  Add the some (volunteer) subscribers at the problem site and a few others.

4.  Send mail to the new list (does not have to be weighty stuff … test 1, test 2 etc are fine -- does have to have a "Subject:" though)

5.  Is the new stuff rejected?  If so, it is your site, not simply your list.

(oh yeah, don't forget to delete the test list when no longer needed)

Solving (or even correctly identifying) these is not always straightforward.  I recall a lot of owners and admins threw in the towel when the mail header problem came up.  And not without reason.  If resolution is not possible (can happen) do what you can to notify the subscribers of the problem.  Don't try to assign blame -- won't work and you may be seeing something that will takes months (years) to undo.

Again, best wishes on resolving this.
--Joe

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