Handle Received header fallback for missing Delivered-To

What does this MR do and why?

Due to a change in Google Groups handling of emails, Delivered-To is no longer being included in some (most?) emails that are arriving via a Google Group and this is disrupting Service Desk. This MR allows another fallback to read the Received headers that includes the destination email.

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Related to gitlab-com/gl-infra/production#6419 (closed) and https://gitlab.com/gitlab-com/gl-infra/production/-/issues/6421