From what I understood you need statistical tests to detect the watermark. You cannot easily detect by eyeballing that a coin will land on heads 55% of the time instead of 50%

https://en.wikipedia.org/wiki/Ambigram

What is wrong with WireGuard?

I’m extremely satisfied with mailbox.org!

They support all standard email protocols (IMAP, POP3, SMTP) even Sieve for server-side filters, so you can use any mail client you prefer rather than being locked into official apps. They offer CardDAV for contacts and CalDAV for calendars, and their drive can be mounted via WebDAV.

Pricing is reasonable and personal domains are supported.

Isn’t Tuta based in Germany anyway?