Mail addressed to
"john at cs.cmu.edu"
was ambiguous, but was delivered to
John_G_Dorsey (john)
The name "john" also matched the following users
Full name (login name)
Andrew_John (aj14)
Bonnie_John (bej)
To avoid this warning message in the future, address the mail as
"John_G_Dorsey at cs.cmu.edu"