Actually, two emails a month is the upper limit, which we use for actively engaged new signups. With 2 emails a week in this category, we were able to get about 4 times more engagement and time spent on the app than single email a week.

Also, we measure when people use the emails we sent and it's generally of no use to send more emails, after two consecutive emails not being clicked on - so one can stop there.

I don't see why one should not send two or even more emails, when the user wants them. Users have understood our product better and it helped achieve what we define as activation, much faster.

On the other hand, If you were a normal trial signup who just registered and never actually used the product, we'd email you thrice in the first month (including the first email) and then once every 3/6 months. We're also thinking of sending some informative blog posts once in a while. This system isn't old enough yet to give significant data about the effectiveness of the mailers sent after the first month.