This feature has been released. If you to MassMailer Setup -> Utilities you can manually delete the stats email status, activity history between a date range or schedule an auto delete that are older than a set number of days.
Thanks for the feature request!!! This will be a nice feature which will enhance Massmailer.
I will keep you posted once this will gets implemented.
Mansa Support Team
A configuration setting to enable automatic deletion of email statistics would be helpful. Let the user setting up MassMailer set a timeframe for how long the statics should be retained. Once that time frame has passed delete the data.