[Fix] Forged recipients: compare addresses as they were transmitted

[Fix] Forged recipients: compare addresses as they were transmitted

The rule compared the rewritten envelope view against the wire MIME
headers, so a recipient alias mapping to another domain (legal for
recipients) could make a perfectly matching To header look forged.
Compare the envelope and the headers at the same altitude: both as they
were seen in the message, which is also what this rule checked before
alias rewriting was applied to tasks.

Issue: #6137