The mailing list for listmasters using Sympa

[address@concealed]

Thank you!

This got me on the right track. In case anyone else has this situation, this 
is
what happened and how I fixed it:

Somehow, the line in my exim.conf that listed sympa as a trusted user was
overwritten during an exim update. Once that was corrected, mail to previously
existing lists went through without a problem.

Then, for the newly created lists I'd just added (which was why I had changed
the listmaster in the first place) I just had to manually add the aliases to
the /etc/valiases/listdomain.xxx file.

And that was it. Now everything works fine.

Thanks again for pointing me in the right direction!