The instructions for using RecoverDeletedMessages are given here:

https://freeshell.de//~kaosmos/index-en.html#recDelMsg

First, you will be prompted to go offline (File/Offline/Work Offline), then right-click a folder, Recover deleted messages, and the recovered files will be placed in a new subfolder, Deleted_emails_0.

Thanks a lot for your answer, but when right-click the folder,I got the following message:

Unsupported type folder ( newsgroup or Imap not set for offline use )

Regards. J.Pierre

According to the add-on description, "it should be able also to recover deleted messages from Imap folders, if they're set for offline use". Right-click the folder, Properties, Synchronization tab, and check "Select this folder for offline use" and click Download Now. Once that is done, you should be able to recover deleted messages from IMAP folders, but it may not work if the messages were deleted before the setting was changed.

Also, check the Trash or Deleted Items folder for the account to see if the message is there.

Thanks all messages recoverd now. Rgds. Jean-Pierre