I think that pmail looks a bit to much like gmail (possible errors). That + the funny connotation for english speakers.
Not sure about it..

proton.ch is definitively better. But proton.com is used by a car maker. (possible mistake again ?)

I agree, @pmail.com might also persuade more people to move to ProtonMail from Gmail.
Some users mention in English that the "p" has social connotations, but this is a ridiculous argument from anyone over 5 years old.
Yet, if the "p" bothers those weak of heart, maybe @ptmail, @promail, @protmail would also be short and acceptable.

Even if I agree that "Protonmail" has a strong sound to it, the idea is not bad.
" @pmail.ch ", " @pmail.com " should be available as a choice in the preferences for every user.

I would have to agree with User, Pmail sounds funny to me. if you have to shorten it, call it ProMail. However, I like Proton as well.

Proton does have a strong sound to it. Leave it!

Pmail would have some unfortunate connotations in English if pronounced phonetically.

P makes a sound that is the same as the word "pee", which means "urine".

This presents obvious social difficulties with using your account as a general purpose email address for obvious reasons. :)

@protonmail.com is fine. If a shorten name is desired by some, please at least choose something other than pmail. :)