I think (repeat think!) this kind of exists - as long as the simple login address has received an email.

When I have just replied to an email sent to one of my simple login addresses/aliases, it said it was about to be sent from my primary proton address, which I was hesitant to do but sent it anyway (it was a company I trusted).

I have just received a reply from the company and the email receipts appear to have maintained the simple login email address.

Admittedly this isn’t the ability to send a brand new email from simple login alias but it seems (unless I’m missing something in the interaction I just had!) that simple login aliases do get maintained once a company has sent an email to the hidden address.

Please correct me if I’m wrong or missed a point there though!

EDIT - apologies, I’m on a rubbish internet connection and the later replies hadn’t loaded - they are much better replies than mine!

I will not use an email service that does not allow me to use aliases.
FastMail lets me use an alias.

I have an @Linux.com alias that I paid the Linux Foundation for. I shouldn't have to own the entire Linux.com domain for ProtonMail to let me send emails with the alias I bought.

This is the exact reason I signed up for ProtonMail, however, am using ProtonPass. Very disappointed! I left a service that allowed 25 alias that I could send email from for a downgrade of 15.

This should be the core selling point of Proton Pass + Proton Mail combo. I'm suprised we cannot use this at all...

This is what I signed-up to mention.

I hate that I have to choose between using `+alias`s (that I can actually reply with, but leak my email to anyone paying attention) and simplelogin aliases (that do a better job of hiding my email, but I can't reply with, so end up leaking my email if I need to every reply anyway).

If my aliases from Proton Pass were just options to use when sending email, that would be an amazing choice. (Maybe show them in "Identity and Addresses", with the other ones?)

Am using the SL extension in Brave so when replying to an SL aliased email from proton web it does indeed auto populate the "TO/Recipient" field with the SL reverse-alias address.

Exactly, why give unlimited aliases through Proton Pass if it’s not easy to send an email from that alias from proton mail? For instance, I might have subscribed to a clothing store with an alias, and later decide to email them from given alias. It should be easier.

critical......

Yes. This is what Fastmail excels at. Fastmail is also able automatically set "Send From" address to the alias where it received the email originally without the need for user to change it manually.

Agreed. Emails received from a simple login forwarding alias should default to the simple login alias in the 'Sent From' field if I reply to the email.

Maybe I'm misunderstanding the question, but isn't done using a reverse-alias?

Specifically, under "Aliases" in SimpleLogin, find the alias you want to send as then click "Contacts", enter the email you are sending to in the textbox, "Create reverse-alias" then copy that reverse-alias to use as the recipient in Proton Mail.

Agreed it's rather convoluted though (it's simpler if you are replying to an email, as SimpleLogin automatically replaces the actual senders with these reverse-aliases), and it would be great if Proton Mail had more seamless integration with SimpleLogin (e.g. select to send as alias which would then auto-create/substitute the reverse-alias).

Strictly speaking, there is a way to do this, but it is a bit of a pain in the neck.

You can reply to an e-mail that came to an alias and it uses that address in the reply.

So send yourself an e-mail to that alias. Then whenever you want to send an e-mail using that aliases as the sender, reply to the mail you sent yourself and change the title and recipients name and get rid of the text in the message you sent yourself.

Try it. Suppose your address is example@proton.me. Send yourself an e-mail to example+3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117067982148086513282306647093844609550582231725359408128481117450284102701938521105559644622948954930381964428810975665933446128475@proton.me. Then use reply to send a message to a friend.

Note that one downside to using aliases is that the PGP encryption no longer works. Each '+' alias you use would need its own PGP key. If you could identify '+' addresses that you would like to use for encrypted e-mail, it would be nice to be able to select those '+' addresses and create a PGP key for each of them.