I'm curious about how they'll handle SMS. I once worked for a place that self hosted a message gateway by co-locating a bunch of cellphones alongside their servers. It worked but probably wouldn't scale for their purposes. Our setup also assumed that customers were in the same country as the servers.

Email service providers?

What’s that?

sudo apt install postfix

Configure DMARC and SPF and you’re fine.

Yes, it's as easy as those two commands, unless you want people to actually receive your emails.

It is not that simple yes, however it is not too hard, especially if you are mostly only sending to registered users. It is lot harder to setup a dc and get ISPs to peer with you than setup the app itself.

Do you have some resource to share on this simple set up?

There are one or two projects like postal[1] or mailbox[2] that make it much easier than it was before.

[1] https://github.com/postalhq/postal

[2] https://github.com/mail-in-a-box/mailinabox