A new version 2.84.2 of Simple Mail is available on AMO.
https://addons.mozilla.org/en-US/firefox/addon/simple-mail/

What's new in this version ? :
- FIXED : Bug with Spam management
- FIXED : Refeshing mail :
-1 : No automatic refresh and no refresh clicking on refresh button, refresh only by clicking on account name
0 : No automatic refresh but refresh when clicking on refresh button and on account name
+10, +20 : Automatic refresh, when clicking on refresh button and on account name
- ADDED : Now, you can't add two addresses with the same email in the address book
- FIXED : Attach multiples files don't work
- ADDED : Popup when you send a mail
- ADDED : After send return to the SimpleMail window if multiples tabs
- FIXED : Bug with acknowledgements