EMail not saving messages

As far as I can determine I am still with Cox email.  Today two out of my six accounts stopped saving messages.  I use Thunderbird for emails and the 'SENT' file is grayed out and no messages show. 

I get two error messages 

   Your message was sent but a copy was not placed in your sent folder due to network or file problems

   The current command did not succeed.  The mail server for account  xxxx@cox.net responded [TRYCREATE] Mailbox doesn't exist.

When I log in to webmail.cox.net, I can see the old message but the newly sent message are not being saved.  Four out of the six account are working OK