[QUESTION] How do instances that use platforms such as Lemmy send verification, password reset, etc. e-mails? Do they use SMTP or some external service that provides an API?

No, it's just a service that's running without me thinking about it.

My setup is:

• debian as system
• exim for smtp
• courier for pop3
• mariadb for accounts

But I'd like to make a point that's not being made in any of the other comments. It does not require an SMTP server to send e-mail. All you have to do is lookup the MX DNS record of the domain, connect to that SMTP server and write a few commands fx.:

EHLO senderdomain.tld
MAIL FROM:<yourmail@yourdomain.tld> 
RCPT TO:<recipient@recipientdomain.tld>
DATA
Subject: Blabla

Bla bla
.

Yes, it is.

I've been running my own mail server for decades now (a quite odd hobby, I know) and that's not to be recommended for anyone who doesn't have a particular interest in e-mail. SMTP is from the early 1980s with roots in the 1970s and has had layer upon layer bolted on since then. It's a fantastic mess.

I self host a lot of services, but I will never self host my email server If any of my self hosted service would have to go down, I will work on it as soon as I can, but it coudl that it's not that soon. If I would have self hosted my own email server at it would go down, I would have to work on it straight away because usually after 24/48 hours the other email server give up and send back the email (that was directed to me) to the sender, I could not receive airplane ticket for example and a lot of other stuff. What if that would happen when I'm on holiday in the other side of the world??? 🙈

@Asudox once I set it up *correctly* I didn't have problems yet

But big email providers like google like to not deliver emails if you do stuff wrong. I'm not self-hosting for my personal email stuff, but I didn't want to pay or put my email pw on my server, so I had no choice (i could've created a new email account, but i didn't want to put a 3rd party in the middle)

I'll counter the other comments. It's not any more difficult to maintain than any other service. What is a pain is the spam, and as I run it for a business the users. The biggest problem I have is other mail servers been incorrectly configured for SPF, DKIM and dmarc , usually it's things like external billing platforms that have been integrated incorrectly by customers and suppliers. Spam is now starting to come though with all the proper headers for SPF and DKIM