The WTFPL is risky in certain jurisdictions, as it does not have a NO WARRANTY clause.

Wiser alternative:

Do whatever you like with the original work, just don’t be a dick.

Thank you! :-)

Again, I’m talking about the server part here, and there is a lot preventing a server to be both a web and a mail server.

You understand that web servers (listening on a web server port) and mail servers (listening on one or more mail server ports, possibly on the same computer) are entirely different technologies?

JMAP sounds interesting indeed, but as far as I understand, there is an underwhelming number of clients that speak it?

A website is the response a web server sends on a web port to a web browser. SMTP on port 80/443 won’t work well, but please try.

If you try it, report back. ;-) My current setup is mostly OpenSMTPD & Dovecot, but I’m open for good reasons to move away.

Websites do not have the functionality to connect to mail servers. These are different protocols.

For mail server infrastructure, Stalwart is said to be pretty good. I haven’t had a chance to try it yet.

A few thoughts on this:

• Debian is, in my opinion, oversized for an OPZ2. If it absolutely has to be Linux (does it?), Alpine or Void might be worth a closer look.
• Why SFTP? Wouldn’t SCP be enough?
• Automatic updates are risky for a device that is supposed to run always. Instead, I would recommend sending update notifications and then manually applying an update from time to time. If the device no longer boots up, you often don’t even notice it.