One of my email addresses suddenly stopped working. It turned out that "Hosting Provider 3" was shutting down, and suggested that users migrate to "Hosting Provider 1," who unfortunately didn't support DynDNS, a feature I need. I considered migrating to "Hosting Provider 2" (which I am already using for other things) instead, but this was not possible since they don't support my Top-Level Domain (TLD). The solution? Migrate the domain registration to 1 and configure the domain to use 2's name servers.
If you've read my text about my ISP Telia, you know that I dislike them and their firewall. Because of them I cannot receive or send emails from my home network. My email provider doesn't support custom domain names in email addresses, but "Hosting Provider 2" has an email relay that can forward incoming mail to my inbox. To send outgoing emails from my own domain, I tell my email provider to send emails through my home server, which forwards them through the relay provided by my ISP, bypassing their firewall.
This was a brief summary of my setup, parts of which are illustrated below. Both the ISP and "Hosting Provider 2" relays are completely undocumented, and I've spent many days experimenting to understand how they work. Truth be told, "Hosting Provider 2" doesn't really support DynDNS either, but I reverse-engineered their API ("Hosting Provider 3" requires CAPTCHA for login). The actual setup includes VPN, additional firewalls, and varies between my different domains because they have different registrars (due to TLD restrictions). It's a miracle that it works, albeit taking 15 to receive and deliver emails, most of which end up in the spam folder anyway (yes, I already use SPF). But this way, I only pay for domain registration and DNS and still manage to bypass my ISP's firewall. However, if my email breaks again, it might be time to consider paying for Protonmail.