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-).