anything less than once per month is no longer a regular eblast update. The emails start to get so far apart that you’re losing most of the big advantages of email marketing — you’re no longer keeping your company in the forefront of customers’ minds.Certainly if you have some kind of truly important news you could send out a mass email about it, but it’s not really a newsletter at that point. It’s just a random email sent out to customers.The once per month point, however, can be very effective. It’s just inside the timeline so that your customers will remember you, and when it comes time to make a purchase decision, you’ll be in their head.