I'm trying to figure out how I can send out all of our invoices we set up from an email address
But that email address is an email alias in Gmail

For example: The main user account in Gmail is firstname@domain.com
But in Google workspace I linked invoices@domain.com to the original user's account as an alias

However, if I try to set up the email outgoing server and email alias, I have to use the firstname@domain.com username and password or the connection is invalid.
Now all of the invoices I send out are being send from firstname@domain.com.

Not from invoices@domain.com. But here's the thing: the account I made in Odoo, I set up with Invoices@domain.com...

Furthermore, if I want to reply to the email, replies simply don't come in? 

It's a catchall... However, replies should be send to invoices@domain.com, which have to come in in to the mailbox of the user 'firstname@domain?com'

Can someone please help me? Been reading and searching for 2 days now, can't find anything about Gmail alias addresses and unfortunately no response to tickets or any help from Odoo itself.