We are currently using Usermin (on an external MTA) to Autoreply (when on Vacation for example).
Usermin gives us the ability to change (or add) certain headers.

As an example things that we use are :

- changing the Subject in the autoreply
("Subject: Out of Office Autoreply Re: $SUBJECT")

- Changing the "FROM:"

- And Changing (adding, really) the "Reply-To:"
("Do NOT Reply" <noreply@mydomain.com>)

I have not been able to find out whether it is possible
to do the same with the in-built Zimbra Vacation.

One thing that particularly bothers me is that (unlike most Vacation messages
I receive) the recipient does not immediately notice (from the Subject) that the email is an "out of office" auto-reply.

If anybody has a solution, may they point me in the right

In theory every user sets his/her own headers but in practice they all
copy from a template. So a "general solution" affecting all the users is also acceptable.

Thank you.