[lassejansen]

One great application of this would be to hide the unsubscribe link if the email is forwarded.

[cookiecaper]

Would only work if the recipient you forward to has a different email provider.

[anonbanker]

no anti-pattern is ever "great".

[AdamTReineke]

I wouldn't call that an anti-pattern, it prevents people you forwarded the mail to from easily unsubscribing you from the mailing list. They aren't subscribed, they shouldn't be able to unsubscribe.