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I think rsync is short for remote sync

Yes they're usually called " Display". IIRC Display variants are optimized to be used on digital displays (usually on the web), where a lower resolution (72ish DPI) than printing (~300 DPI) is quite common.

I fail to see how the mails being encrypted stops them from using IMAP(s) like everyone else. IMAP doesn't care what the contents of the email it's sending/fetching are, and is perfectly compatible with other E2EE solutions like PGP/GPG which they say their solution is based on.