I have a buddy in email marketing. Pick an eCommerce company between 50-400 mill/year revenue and he probably knows the company managing the email lists, capture portals inboxes and patterns. They have an average roi of something like 300%. I don't remember more exact numbers but it's asinine. They claim an average across the entire company is about a 40% CTO rate. They have meetings and confidences, in public, calculate where to but the close (X) button on a form every 6 months so that websites get maximum user email capture. Users get used to where the buttons are over time and some websites train people differently. They often close the offer out before reading it, and if you make it so that they feel like it won't go away without entering their email, they won't, and therefor the company is missing out on a returning customer XD Im told they have monthly industry meetings to calculate compliance costs. Like, - Do I have to make a working unsubscribe button if the user is in X state - What happens if we wait more than 30 days to remove them from the list if they've already unsubscribed - They said the _link_ has to work, it doesn't say the button has to work - California requires unsubscribe actually work, but no other states do... - After how many emails per day so the user CTO drop? Last year I was told for most eCommerce segments about 3 emails/week was peak, any more and it fell off. Sometime this summer I was told it's gone to 2/day is peak. One when users wake up, and one in their evening. These things are a calculated science. If you want to know what malevolence is going on in the world, find someone who's trying to sell it, they're not hard to find, and they will brag all about it.

One per day is more than enough to drop me. But I also actually care about the number of notifications I get and inbox hygiene. One per week is the most I care to get from a single source (other than security related things like card transactions).