Well its a long complicated story. I’ll give you the super short version.
Google App Engine has an API for sending email. So for the first 4 games we made on app engine we could send as much as we want, we just paid for the CPU and bandwidth. There was a rate limit but we never got close to hitting it. Basically free.
Then sometime between Triton and the new Blight, all new app engine instances had a quota of 100 email a day imposed, and you had to ask google to increase the quota. We asked on Blight but basically received no response for 3 months. Then I reached out to a contact I had inside google and had the quota increased to something reasonable like 20k a day. (Keep in mind if we get featured on Apple I would need to send this many email address confirmations let alone daily turn emails)
Then late last year Google told us they will no longer be increasing email quotas for people and that we had to go find a third party to send email. bummer. We had to use a third party for Colossus (BBB)
I looked around and settled on Mailgun. I liked the site and the API. As a google customer we get 30k emails free a month, then $0.00050 per email after that. I know that doesn’t sound like much, but my guess we need to have around 2k daily active players for the game to be successful. 2k Players * 30 days * say 2 emails a day is 120K emails. That’s only $45 a month - but we don’t know how many of those 2k player will be paying, or how many games they are in, or how long free players will stick around for - or how long paying players are going to play for for that matter. There is at least 1 BBB player in 40 simultaneous games.