Mail being sent to my list (call it MYLIST-L) is being rejected with "unknown mailer error 137", i.e.,

  ----- The following addresses had permanent fatal errors -----
"|/usr/local/bin/lsv_amin -t MYLIST-L"
   (expanded from: )

  ----- Transcript of session follows -----
554 "|/usr/local/bin/lsv_amin -t MYLIST-L"... unknown mailer error 137

The most likely reason we've found for this so far is that someone may have updated /etc/aliases without following up with 'newaliases'. If you check the syslog you will probably come up with something like this:

Sep 17 03:13:03 listserv sendmail[1434]: alias database /etc/aliases.db out of date

If you run 'newaliases' it is probable that the error will disappear. Otherwise a reboot of the machine may be required.