How to setup a vacation message with Thunderbird reading hermes email
You'll find the information you need at
http://www.cam.ac.uk/cs/docs/infosheets/is12/In summary you open a web-browser and go to the management interface for you email account at https://webmail.hermes.cam.ac.uk, select account management and vacation message. The above infosheet describes this in more detail.
Note. This page only applies to people who use hermes email. DAMTP users should add an alias to the hermes webmail vacation settings to make emails sent to crsid@damtp.cam.ac.uk send out a vacation message e.g. b.smith@damtp.cam.ac.uk NOT bs123@damtp.cam.ac.uk - this gets done already.