Rogue email mailboxes being auto generated and populated with spam

I've searched thoroughly for this and can't seem to find anyone having this issue. On my devices using Apple Mail, I am having spam email not even addressed to me appearing in auto created foldes using 3 digit numbers as the folder names. I can't delete them all and a few days later can have up to a dozen or so again. This is on an @domain email account and the email is not passing through the server. I have the email account set up on 4 devices as IMAP, so they are all synced, and can also access the mail via a webmail app. In the webmail app and one computer I am using Thunderbird, these mailboxes are not showing up. They are only populating on the 3 devices where I am using Apple Mail.

They seem to be separating based on source. For example, one folder may have emails from "Miss Lola" and another "Loog Guitars" and then another all Chinese writing. The "To" email address is not valid and will look something like keith+123@mydomain.com ("mydomain") will be my actual domain) then it puts it in a created folder "123"

I have no idea where they are coming or how to stop it. It is concerning because it seems like Malware activity. But nothing shows and a scan.

Has anyone run into this? How can I stop it?