| When you set up vacation there is a list of addresses (vacation addresses) that you want to trigger replies. A vacation message is only sent when one of these addresses is in the To or CC list of a message, so you don't reply to mail from a mailing list.
The first address in this list will be used as the From adderess in the reply
By the way -- the sender of the vacation message is '<>' the magic address used by the mail system for information messages. Some broken mail systems do not accept mail from '<>' so not all vacation messages will get through
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Accessing Services at OA5 / YASK / Irial : 
Vacation is using a really strange address as the From in replies