Add an ad button in protonmail(in the settings) that allows users to choose if they want to see ads.
There could be options added that allow the user to choose what type of ads they want to see for example tech ads or clothing ads and so on.
Just think of what this could do for your revenue! You are not crossing any lines as the option to see ads is up to the user.

Have nested subfolders (within folders). I have this feature in all other mail apps I use

bring back message mode.

surely your users have already indicated this requirement,
and as others have expressed better: if the base system doesn't work for the client, they prolly wont move to the paid service...

I tried settings, but did not find appearance

Need a calendar

Im wondering how to get involved. I dont have much knowledge on web except using it. I am very interested in working for ProtonMail. At any lowest entry level if any. Or perhaps a course or class of some sort could be added into the careers option?

One huge step towards privacy is to have disposable burner credit cards, or virtual cards.

https://privacy.com/ does this in the US. AFAIK today (2018) no-one is doing that in Europe.

Add iPhone to phone options in contacts

It would be great that we can apply newly created filters on the mailbox.

In order to get people to switch from one service to another, Proton needs to offer the security it is revered for as well as other services like Document editing, file storage, calendar operations, etc. Google does this without the security and trust, so if Proton could develop these apps and integrate them with ProtonMail, it would be a serious rival of Google and Yahoo!

Please make ProtonMail APK for F-Droid, because we love open source.

Even though the Protonmail mail-server supports TLS 1.2, the cipher suite configuration is suboptimal. It is highly recommended that the server is reconfigured so that the cipher suites providing forward secrecy (ECDHE or DHE in the name, in this order of preference) and authenticated encryption (GCM or CHACHA20 in the name) are at the top. The server should also be configured to select the best-available suite.

This should be fairly easy to implement and provides a much needed security-boost.

could you PLEASE offer a choice of BIGGER fonts for our messages, instead of the microscopic font you offer. do you pay by the font size

Show the difference between a message that has just been received via SSL and a message that was encrypted using your public key.

Currently there is no way how you can see if a message was encrypted using your PGP-Key, when send from the outside.

Checked help, does not pull up anyting. Dug through features, nothing. Kinda of a basic feature.

Release nicks of abandon accounts to be use for new people

GHD SPORTS APK used to watch new sports matches on android device and yoou can watch more videos , highlights and score...

When I sign into ProtonMail in a browser, you show a whole bunch of promotional ads on the right. Could we instead show folder count, storage breakdown, other meta-data (system status) with a link 'what's new'

Quite tired of being told 'ProtonMail is the highest rated email app in the App store"