Hi,

I too am looking for a solution to this:
I want to be able to setup a forwarding mailaddress (me@example.com) that would allow using it as an SMTP but again, without the mailbox (as it's all being forwarded to i.e. gmail)

That way i could have a gmail account, receive my me@example.com mail there but also send it from there via Plesk to the outside world. Thus the need for an SMTP credential (me@example.com / password) but again without the local storage of incoming mail...

(this is basically a copy of https://plesk.uservoice.com/forums/184549-feature-suggestions/suggestions/5192375-smtp)

The usability of a mail forwarder is degraded.
problems
One the sending side:(read for google hotmail and others)
sending from google with the forwarder as the from adress needs to be authorized by the smtp server(for the domain of the forwarding adress)
In plesk you can not authenticate on a smtp server if you have a forward only address.
on the forwarding side:
All mail forwarded by a Plesk forwarding service, even when you have a mailbox, are ending in the spam box when a receiving mail server checks for the SPF authorized server that is because the Plesk forwarder does not use SRS to transform the address fields.
This happens when you do not implement SRS
the authorized server is google because the from address is a google address and the sending server is the Plesk forwarder. the google spf rules are "drop the mail if sender is not google" so the mail does not even have to finish in the spam it can be dropped after spf check (and that is the first action of the mail receiving protocol.

here are 2 examples of solutions:
1a implement smtp authorization also when forward only
1b implement SRS in the forwarding service

2 create the possibility in a active mailbox to forward and delete in one action so the mailbox stays empty

Hope to hear from you.