You need all the clues that you can find: did you display all the hidden
headers e.g., RECEIVED and RESEND headers?

Did you do a SCAN of any partial name or mail address?

Was the date of the email message recent or old (and thus the person had
already unsubscribed e.g., I just starting to receive bounces from mail
generated 3 months ago to a then-subscriber).

/Pete

PS followup to lstown-l only