Consider the following scenario: Proton user U creates an alias for company X. Sends the alias to person A from company X, exchanges a few emails. So far all good.
Then U has a phone call with person B from company X or Y, receives B's email address, and adds this address in Cc of a reply to A, not realising a reverse alias needs to be created first. So now A receives an email with the alias in the "From:" field, and B receives the same email, with U's main email in the "From:" field.
B replies to all, now A has U's main email address...

I see 3 possibilities to prevent this scenario from occurring:
a. Easy implementation: use different colours for reverse aliases and normal email adresses when an email is being composed. For example red would indicate that the recipient will see U's main email address, and blue could be used for reverse aliases (please no green here, mind the colour blind).
b. More work: add a setting for preventing main email leaks, so a warning pops up when clicking send if the main email is about to be leaked, with "Continue" and "Cancel" options in the pop-up, where "Cancel" would be the default choice. There could be 3 settings for for controlling when the warnings appear: "Never", "Only when an email about to be sent has reverse aliases and normal addresses", and "Always" (default for new users).
c. Most complex, but most user friendly: instead of just warning, the pop-up could offer to create a reverse alias for any new address, or use an existing one if the contact already exists in an alias (handling of same contact in different alias(es) would make this complex).

As the number of Proton users is growing, it is important to consider the non-Geeks who are unable to read, understand and remember instructions.