I get an error message when sending an email, and it will not let me save the sent message.

I get an error message when sending an email, and it will not let me save the sent message, the emails are still sending, but they do not save, and I have to select the 'cancel' option when I get the message 'there was an error saving message to sent' - as pressing retry just repeats the cycle. I haven't changed anything on my pc, it worked when I left work yesterday, and today it doesn't work!