2. Wrong Username
If the user part of the email address (the bit before the '@' sign) is not recognised by the remote system, then the error message you get back will depend on the client you are using and whether the domain you are emailing is an Oxford domain or not.
This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: firstname.lastname@example.org SMTP error from remote mail server after RCPT TO:<email@example.com>: host cluster1.eu.messagelabs.com [22.214.171.124]: 550 Invalid recipient <firstname.lastname@example.org> (#5.1.1)
Delivery has failed to these recipients or distribution lists: email@example.com Examine the text below for diagnostic information returned from the remote server. If you have further problems then please contact firstname.lastname@example.org. The following organization rejected your message: relay5.mail.ox.ac.uk. _____ Sent by Microsoft Exchange Server 2007 Diagnostic information for administrators: Generating server: exht01.ad.oak.ox.ac.uk email@example.com relay5.mail.ox.ac.uk #550-RPT-UNK: Recipient unknown. Search at http://www.ox.ac.uk/contact/ 550 http://www.oucs.ox.ac.uk/network/smtp/relay/index.xml?ID=errors#rpt-unk ##
The same reasons for possible failure apply as for errors with the site part - mis-typing or transcription, incorrect original reference and the username no longer being valid. Again, if you can spot the problem immediately, simply re-send the message with the correct address. If not, then you will need to go back to your original source.