550 username... User unknown

Cause

This sendmail(1M) message indicates that the intended recipient, specified by the portion of the address before the at-sign (@), could not be located on the destination host machine.

Action

Check the email address and try again, perhaps with a slightly different spelling. If this does not work, contact the intended recipient and ask for a proper address.

See Also

For information on how sendmail(1M) works, see the System Administration Guide, Volume 3.