This add-on works okay, but there are a couple of things that would make it better:
1) Play the sound WHENEVER it checks and finds an unread email in the inbox. Playing a sound only once kind of defeats the purpose of an alerter - if you aren't within ear-shot of the alert, you will forever miss it.
2) Make the sound that is played for an alert user selectable. Just give the user an option to select the sound that he prefers.
3) Make type of notification an option - allow a user to select popup notification, sound notification or both popup and sound.

I tried to change the Check interval (in seconds) to 60 and it keeps going back to 300. Would be nice if this could be changed to a lower value.

Hi man the add on just shows a sand watch icon and never checks for new emails. So
the interval was 300 sec. than I changed it to 60. Nothing. Still that glass watch icon