Your requirement for a mobile iOS app makes it harder so I’ll go non-free software with my suggestion: Tap Forms. Offline first, iCloud sync, macOS and iOS apps. But no Android or Windows apps.

If you wanna keep it self-hosted, these services need an internet connection anyway, even Airtable. Just go with a Web based one that has good mobile layout.

In case nginx does not work out for you, you can opt for more generic reverse proxies. frp is my favorite one: https://github.com/fatedier/frp

See https://github.com/anderspitman/awesome-tunneling

I have self-hosted my own emails many times. Up to having three SMTP servers with failsafe option at DNS.

It’s super nice, but I would never self-host SMTP again. It’s a nightmare. I had to email or open a ticket at most ISPs despite my clean IPs. Most ISPs simply blacklist all IPs unless they are major email providers already.

My advice is go for it but let SMTP be handled by who will deal with these frustrations. MXroute is a great choice and it’s cheap.

I use https://github.com/bastienwirtz/homer because it is dead simple to configure, and I protect it with HTTP auth at the reverse proxy level.

In some cases outbound only, in other cases inbound, too, with redirect somewhere else. I’m not using them to store emails right now.

MXroute. First, because Jar is stupid (hope someone will get the reference). Second, because they are awesome and cheap at the same time. You can go from full-fledged hosting with them to using them as relay, and for pizza money for a year.