Use email worker for autoresponder on email routing

I use email rotuing for my very small domain (3 emails per week, a comunity service with almost a zero budget so - i eate all free tiers i could have).
I setup a [email protected] (on CF email rouging) → free gmail account (15 GB free) it works. I do need a working autoresponder like “your email is important to us”.

CF adds an envelope to every email (and, well it does it by the book, state of art and every piece of RFC is respected), unfortunatly in GSutie - autoresponder reponds to evenlope email - like
SRS0=-c[email protected]

(if i simply go with respond on gmail it does fine, it’s only with autoresponders).

So - the real chalange is email worker able to do a autoresponder (my Gmail smtp is ready ! and also i have a free tier other smtp service to sent my emails, so i don’t want CF to be SMTP - simply add some static text - ‘your email is realy important to us, please be paitent’ and - use - other smtp to do autoresponse).
And still - forward mail to Gmail as it does already.

Any hints?