Cannot send emails. Can't process message.

When I try to send an email from eM Client on my laptop, I get the following error message in a pop-up box:  “Can’t process message.  Couldn’t process the message because the following error occurred. Consider selecting a different account to send messages from.  Folder has been deleted.”  I have tried sending from another account but get the same message.  I can send emails OK if I log into my Gmail account on the server, and they reflect in my Sent folder on my laptop.