http://privacyy3tsy4mge4qmg4nsid2vnhl7szzupphhkfsxvayx5tl2ztbqd.onion/protonmail/propaganda-unencrypted.html
All this is done in memory so that by the time anything is permanently stored to disk, the email is already un-readable to us. This gives us a very limited window to perform spam filtering on incoming messages." Source: https://proton.me/blog/encrypted-email-spam-filtering Then they further elaborate, "Secondly, the message is passed through our customized Bayesian filters which marks suspicious messages as spam.