[IfOnlyYouKnew]

Well... Protonmail happens to be the only e-mail provider that cannot fully be used as intended using just NMAP, because the core feature is its proprietary encryption and related features. So I would guess it's probably a singular outlier in having more web client users, rather than less.

[noisem4ker]

Nitpick: I think you meant IMAP (a standard email client protocol) instead of NMAP (a port scanning program).

[tlavoie]

I dunno, there are some pretty neat NSE scripts.

[tremon]

Partly true: you can install a local IMAP gateway (Proton Mail Bridge) to access your inbox from normal MUAs. FAFAIK it supports all regular IMAP operations.