Set a List-ID header on all emails. This can help with filtering in email clients. It will also stop most auto-responders from sending replies back to the sender (out-of-office, on-vacation, etc.)

You can use whatever name and id you want, but the format should be similar to one of the following (including the < and > signs around the actual id):

For a full description of the List-ID specification see RFC-2919.