What we often see with Groupwise vacation messages is the following:

(1) Groupwise user sets vacation message using the Groupwise service.
The vacation message is sent in response to EVERY message received.

(1) Groupwise user receives a message from a LISTSERV list that they
are subscribed to.  Vacation message is sent to the list.

(2) Vacation message from the list is received by the user.  Another
vacation message is sent in response, also to the list.

(3) LISTSERV rejects the second vacation message as a duplicate.  Error
message is sent back to the user.

(4) Vacation message is sent to the LISTSERV address, in response to
the error message.  LISTSERV tries to execute the vacation message as
commands, generating another error message.

(5) The vacation message is sent to LISTSERV again in response to the
error message.  We have a loop.  Normally LISTSERV cuts it off after 50
iterations and serves the user out.  You have to be a LISTSERV
postmaster to see the error message about the user being served out.
After that, further vacation messages go to the LIST owner and the loop
stops.

The loop occasionally will go on forever when the vacation message
happens to have a line that starts with a valid LISTSERV command
(usually "thanks", but also "I" in earlier versions of LISTSERV).  Then
the loop has to be stopped manually at one end or the other.  At the
LISTSERV end, that can be done by manually serving the user out.  We
have a loop detector that picks up such loops by scanning the LISTSERV
logs every night.

We are seeing fewer of these lately.  I don't know if it's because
Groupwise behavior has improved, or simply because some of our big
Groupwise users have switched to Exchange.

Microsoft Exchange doesn't get into loops with LISTSERV with it's Out
Of Office (vacation) messages.  That's because it sends them to the
sender of the message and not the list.  That's plenty annoying when
you are the sender, though.  At least it doesn't respond to every
message received from the same address.

A good vacation program will

(1) Respond to the return-path (SMTP MAIL FROM) address.  LISTSERV will
treat them as bounces.

(2) Respond only once (or perhaps once per week or day) to each unique
address.

(3) Have tests to avoid responding at all to list mail.  Some such
tests are responding only to messages that have the recipient's address
in the To or cc headers -- tricky when users have multiple email
addresses) or not responding to messages with certain return-path
addresses (such as those that start with owner-).