Rated 4 out of 5 stars

Great addon. Is it possible to expand limit of processed messages (now it is 1000)?

This review is for a previous version of the add-on (3.0.0). 

Yes, you can change the pref "extensions.mailsummaries.max_messages" to whatever you like. However, you probably won't see much change by raising this value unless you have a really special use case...