Custom Email: O365 forwarding doesn't add forwarding target to To or Delivered-To header

In contrast to Custom email: Forwarding in Microsoft Exchange ... (#426269 - closed) it looks like if you set up normal email forwarding, the forwarding target is not added to the email header. So GitLab cannot determine what this email is for.

In the context of the custom email feature that means, we have the custom email as the To header (for example support@example.com). The longer Service Desk address (forwarding target) is not mentioned. Right now we only use the mail_key to process emails. We'd need to change that and also look for custom email addresses and +verify custom email verification emails.

Also see in https://gitlab.com/gitlab-com/dev-sub-department/section-dev-request-for-help/-/issues/302 (internal)

We already solved this for Custom email: Forwarding in Microsoft Exchange ... (#426269 - closed) by checking custom email addresses for mail_keys. The current architecture doesn't allow anything but a mail_key being processed, so we'd need to change that or bypass this by fetching the mail_key for a given project when a custom email has been received.