Evolution Error When Sending

[Charlie McVeigh]

I am using Evolution 2.2.3 and FC4.  When sending an email I frequently
(but not always) get an error message pop-up with the following text:

Error while performing operation.
RCPT TO <xxxx at yyyy.com> failed: Requested action not taken:
mailbox name not allowed.

In all cases the email address that I am trying to send to is a valid
email address.  I then press "OK" to dismiss the above message. Pressing
send/receive causes the message to cycle again.  After the 3rd
iteration, my email message is sent as if there is no problem.

Has anyone else seen this error.  Any ideas on how to correct it?

Charlie