It is sometimes required to only have a mailbox in Ispconfig for login purposes. For example: to allow gmail to send email in the name of that person.
But without actually saving the mail in the mailbox, and only forwarding the e-mail.
In order to achieve this, you can use the 'custom rules' in Ispconfig 3.
First make the mailbox, as you usually do.
Once the mailbox is created you can go to "Custom rules".
In that box, enter the following: