550 username... User unknown

Cause

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

Action

Check the e-mail address and try again, perhaps with a slightly different spelling. If this doesn't work, contact the intended recipient and ask for a proper address.

See Also

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