In Seth Godin’s book “Permission Marketing”, he makes the point that being pushy with marketing doesn’t work. In fact, giving the user more choice, can increase their trust.
So if newsletter signup forms asked the user to choose the e-mail frequency. Maybe that could reduce people leaving, since they agreed on the frequency.
Then if user chooses “monthly”, all outgoing emails would be queued on the senders side, and sent out as one batch “recap” style email each month.