Add anchor for incoming email regex, closes 44989

What does this MR do?

Added an anchor for the incoming email regex.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

So that we do not match invalid incoming email addresses like foo%{key}@example.com.someotherdomain.com

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides

What are the relevant issue numbers?

#44989 (closed) - Incoming email regex should be anchored