Properly setting up your own email server is a pain so a lot of us are using email providers (mailjet, sendgrid, mailgun etc). These are easy to set up as SMTP relays but they also cost a lot of $$ if you got serious volume. Right now there is no option (or, at least I did not find one) to disable all chatty email traffic except essentials (registration, pass reset). There is a lot of emails sent for watched topics and private messages. It should be possible to disable these and leave only essential ones.