we had a similar situation... we have a user (user01) who went on leave, and a temp user (user02) is filling in the role, and needs to log into to user01's mailbox with user02's credentials.
We use external ldap as our backend.. so we went around the problem by doing the following:
on user01's zimbra account, it maps to user01 ldap... so we added user02's password as a 2nd userPassword to user01's ldap account, and then also created a user02 ldap account as normal. (so they can log into their pc as themselves)
Then on the zimbra side, we added
user02@domain.com as an alias to user01's account. Because we worked out that, you can log in to an account with either the username of the account, or any alias of the account, and it will match authenticate against the user01's ldap account, and with the 2nd password in there, the temp is now able to log into user01's account using her "username" (user02) and her password (and it auth's against user01's ldap account)
So, because you use active directory, i would say the only way this would be possible if you use adsiedit and edit the properties of the user account and add a second password field to the user..
@phoenix, you can have multiple login credentials for the one account (by using the method above) and it works - im not sure if this will work in AD, or using zimbra's own ldap auth.. but definately using an external ldap system this works.. both the "temp" and the original user, can log into the one account at the same time and with personas, they can sent emails as their own username..
