How much RAM do you have? Do you have swap? How big is your database?
This is the problem that you want to solve.
I have 4GB, which should be more than enough to handle a single Discourse site? Not sure if there is any/how much swap on the Digital Ocean shared plan ($20 a month)
That should be enough unless your database is huge. If you have resized your droplet since you installed you should run discourse-setup again to reset the memory parameters (or edit app.yml yourself).
Do the mail logs in admin / mails /logs / sent|skipped show that the messages are being sent?
Do the mail provider logs show the 15 messages or a bunch more that it hasn’t delivered for some reason?
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