Wow... six years and still nothing? This would be really easy to implement (relatively speaking). I would very much like to the ability to send email from an alias. It is frustrating and limiting when I need to email a company and I cannot email them from the email address they have on file. This has caused problems for me and requires stupid work-arounds. Thank you

It should be possible to choose any From e-mail address, not only for replies (Protonmail should use the incoming mail's recipient address automatically as from-address in replies), but also for sending mails.

Sending should be possible not only from + address extensions, but for custom domains it should be possible from any address, as Protonmail allows us to create a catch-all address.

I found out a workaround:

from your main account send a message to the + alias you want, for example

From: main@protonmail.com
To: main+alias@protonmail.com

When that message arrives, hit reply (or reply all), change the To, CC, and BCC fields, change subject and message.

This is the only thing stopping me from moving and paying for ProtonMail, I don't want to reply with my new, fresh and secure protonmail email address to everyone. I have certain email that I want to reply with that email address and not give them my protonmail email address.

Yes this makes sense.

I was a bit dissappointed to see the implmentation was done this way (as for the use of "+" which is too frequently blocked on forms).

Yahoo is an example implementation of what I think we are after.

it kinda defeats the purpose of having the +alias if you are not able to use it to reply where you used it, example, you want to reply on a customer request ticket on a website you subscribed, their automated system might not recognized your non +alias email if you try to reply within your inbox.

it would be nice to use the +alias in replying to email.. this feature is already implemented at google mail.