sendmail errors

Hello all,

    I have had sendmail set up on my linux box for some time now and haven't
had any problems... just today I had sent an e-mail out without a problem,
but tonight I went to reply to a message and it gave me this error

[Mail not sent. Sending error: 451 queuename: Cannot create "qfVAA13071]

does anyone know what could be causing this, or a way to fix it?
Thanks in advance.

Joel