[Dovecot] Using Dovecot-auth to return error code 450 (or other 4xx) to Postfix when user is on vacation

[IVO GELOV (CRM)]

Hello to all members.

I am using Dovecot for 5 years, but this is my first post here.
I am aware of the various autoresponder scripts for vacation autoreplies (I am using Virtual Vacation 3.1 by Mischa Peters).
I have an issue with auto-replies - it is vulnerable to spamming with forged email address.
Forging can be prevented with several Postfix settings, which I did in the past - but was forced to remove, because
our company occasionaly has clients with improper configurations and those settings prevent us to receive their legitimate mail
(and this of course is not good for the business).
So I have though about another idea. Since I use Dovecot-auth to verify mailbox existence - I just wonder is it
possible to somehow indicate specific error code (and hopefully descriptive text also) to Postfix (e.g. 450 or some other
temporary failure) when the owner of the mailbox is currently on vacation ?

Best wishes,
IVO GELOV