4.2.0 Internal Queuing Error

Over the past couple weeks, I've been periodically getting this message when I attempt to send a message:

An error has occurred while sending mail. The mail server responded: 4.2.0 internal queuing error. Please check the message and try again

The email client is Thunderbird, using imap.cox.net. Some days, it doesn't happen at all. Other days, just about every message I send causes this issue. 

According to the Thunderbird forums, this is a server issue (similar issues over the years have been reported there for Comcast and other providers).

Can you help me resolve this problem? Thanks!