I set up a new account under Thunderbird with an IMAP in-going and SMTP out-going. All mails and folders are correclty imported from the server. I received new mails can delete them (it is updated on the server). So everything looks good. However, when I try to send a new message, the message is sent correctly but while trying to save the mail in the ‘Sent’ folder, it hangs forever.
What is happening is that Thunderbird wants to save a copy of the sent message in a folder called ‘Sent’ which is located in the root of the account folders. If the folder does not exist, Thunderbird will attempt to create it. If it cannot be created, Thunderbird will hang until you cancel the send. In my case, this folder already exists so Thunderbird hangs forever.
One solution is to prevent Thunderbird to save the file since your server is already taking care of that action. So, you need to disable the client to save the file on the server but only locally:
Tools –> Account Settings
Select 'Copies and Folders' under your account.
Look for the box that says 'When sending messages, automatically…' serve that there is a checkbox below it labelled 'Place a copy in…'
Switch from "Sent folder on " + <you account > to the one below that says "Other" and select the "Sent" folder)