The complication comes from the mail server. After digging into my fear of setting up mail server, I realised that part of it resulted from to my laggy SSH connection. I need to edit configuration files interactively and characters not echoing instantaneously makes for horrible experience. To resolve this problem, I set up Mosh recently. Mosh authenticates using SSH and then switches to Mosh which is a shell with predicative echoing. I had to let PuTTY go and set up public-key authentication for SSH in WSL. One may fiddle with the .ssh/config file to add alias and upstream proxy etc. No, I don't remember my server's IP. I need an alias. The by-product is that I can git push in WSL to remote using the exact same remote configuration as what I have been using outside WSL.

Another troubling thing in migration is that I should keep the IP address. It looks that I need to spin up a new droplet, set up stuff, make an image, use that image to nuke the current server and destroy the new droplet.

Finally dkim just feels scary. I am already having a mess with bad naming.

Also I hated my experience with generating a new Diffie-Hellman parameter. I underestimated the time needed and did not screen my session.

Yes, I am still complaining without really starting the migration.