Only allow login with the username/main address, not with every address.
Do not allow that you can log into the account with every address.
If my account name is john.smith then only allow login with john.smith or john.smith@protonmail.com. Not with finance.john.smith@protonmail.com or any other address.
Perfect would be if you would have the choice what address can be used in order to log into your account.
With the current way you have to give away your login username in order to send emails. Hiding the username from the public would be an advantage, since they would have to guess your username and the password. Not only one of them.

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ProtonmailComments commented
Agree with the original poster: I'd like to see logins restricted to one's main email address, or to a username
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Anonymous commented
Username should not be your email.. Look to what fencemail have done..
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Eric Johnson commented
I do something like this on a company account hosted elsewhere. My primary username is a very obscure word. No email ever goes out with that username, but with the alias for the account.
My main reason for doing this is so that if I start getting lots of spam at that address, I can set up and start using a new alias for any new e-mail and set the old alias to only accept e-mail to those specific users I expect to get e-mail from.
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BoC commented
Agree with Joel Drapper
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Anonymous commented
Great solution if you want to separate your private and business lives!
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Anonymous commented
It would be best to separate the login ID from the email address completely. Or allow that option for people who are especially concerned about hackers.
It would be even better if the feature is optional. Just confuse the heck out of anyone trying to hack in!
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JP commented
I'm with Joel. Usernames are part of your online identity and are not considered private.
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Joel Drapper commented
This is such a minimal advantage, it would be a waste of time to develop. Adding one more character to your password would do so much more to improve your security.
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Anonymous commented
f.e.
email: support@protonmail.ch
username: somerandomname
password: somerandompassSo it will be impossibile to login with email address.