Mail Forwarder with static sender address - the chicken-egg-problem

Question

this is a pretty specific case, but it drives me crazy...

We recently migrated our email service to google workspace. We do have an [email protected] address which earlier was configured to forward emails to [email protected]. someinbox is a mail upload feature for tax related invoices of our company. The problem started when i was trying to set up the mail filter in Gmail.

All emails with an attachment should be forwarded to [email protected]. To forward emails with Gmail, google needs to verify that I am allowed to forward to that specific address. It therefore now sends a confirmation email with a link to that address. that email is being sent by eg. [email protected], which is being rejected (550 5.7.1 Security policy violation: sender address not authorized). The problem is, datev only accepts emails from "verified sender addresses". It does that verification by also sending an email verification to that "verified sender address". Which in my case now becomes the [email protected], which I obviously not maintain and therefore i am unable to verify that address.

So I am unable to add the forwarding email address in Gmail, because of the sender google uses to verify the forwarding address.

We use google workspace, so I am able to use the pretty cool routing feature of Gmail. First I created an email-alias called [email protected]. I then setup a rule which simply changes the envelope-sender to [email protected] if the envelope-sender is [email protected]. that part works. If I send an email from the [email protected] to [email protected] it changes [email protected] to [email protected].

The next problem was, every forwarding (which the Gmail filter was doing) works by sending the same email to [email protected] while keeping the original sender. That also happened when I tried to do the same workaround by creating a new email forwarder (or even a mailbox) on a different domain without google workspace. I also tried it using posteo. The original sender is being used as the sender address and therefore datev rejects it. It wouldn't be possible to register all sender address as we get a lot of invoices from business partners.

Does anybody know or see a way of doing this? Aren't there any secure email forwarder which replace the sender address to the one of the forwarder instead of keeping the original one? I know, this is in most cases a pretty nice feature as you can see who the email originally sent, in my case it makes me nuts.