First of all, thanks a lot for your programming effort: Simple Mail it's really a good and useful Firefox extension.

As written from Vishara in Firefox extension site, anyway, it doesn't play nice as a "notify only" tool (like Simplecheck in Windows, for example).
Infact, if you use Simple Mail as a notification system, and use another client to read and delete the mail, Simple Mail doesn't get the message that the mail was deleted and the Status Bar Icon will therefore insist you have new mail until you delete it in Simple Mail.

Now, consider that in Linux universe doesn't exist a notification tool as efficient as Simplecheck in Windows, but Simple Mail could completely reverse this situation. How ?

Simply inserting in "General settings" tab a new option to tell Simple Mail to display, after each mail check, exactly and only the messages existing at the moment on the server, discarding any different information stored into Simple Mail cache.
Otherwise the option could be inserted in each account configuration tab, so user can set individually the mailbox to check in "notify only" mode.

Hoping my suggestion could be accepted, i renew my appreciacion for your Simple Mail.

Sincerely.

Federico Oliosi