As John Gray noted, the functionality exists. From my experience, the functionality works well if the alias was set up in Proton Pass. Proton also owns SimpleLogin.io, which - when I last used an alias set up there - was not quite as seamless.

When replying to an E-mail sent to a Proton Pass alias, the "send to" address will start with "username at domain dot extension" instead of the actual E-mail address. This is the user's only cue that the E-mail will be sent from the alias.

Unfortunately, SimpleLogin.io, which is also owned by Proton, doesn't quite work this way, at least for the first reply. I have found it necessary to login at SimpleLogin.io, find the E-mail address of the sender, and then select an option to copy a reply address to my computer's clipboard - which I can then insert as the "send to" address in a reply.

I agree that documentation is severely lacking on this feature.

And I like the idea of being able to send (or reply) using an alias in the Proton Mail web application. Maybe that idea should be separated from this idea string?

If I can't confirm that the hide-my-email address was used, then the feature is nearly useless. If I need to hide my address, I need to be positive that it's hidden. When I look at my sent mail, it still shows my primary account address. How am I supposed to have any faith that this is working? The feature is just too confusing.

This facility already exists, but it is hidden from the user and there is no documentation on the Proton website (as far as I can find) to explain the mode of operation.

It would be a Good Idea for replies to display the Hide-my-Email address as the default address in the FROM box, together with all the other aliases which could be selected IF required. This would make it crystal clear that, by default, you would be replying using the Hide-my-Email address, which is probably what is wanted.

@parlor807

"So it appears that the definitive say is what @ProtonUser said. The feature already exists, but It should be made clearer. Thank you all."

I have to retract what I just claimed.

I have tested what @Nelethor claimed and it is correct. Replying to an email will automatically have the alias as sender. You can confirm this at the receiver's side. However, if you try to confirm it at your side (as sender) by vieweing the Header, you will see your primary address. This inaccuracy is in the implementation of the Proton Mail client, not due to how the actual header is.

So it appears that the definitive say is what @ProtonUser said. The feature already exists, but It should be made clearer. Thank you all.

@Alan Exactly. What you refer to as "Extra Alias" is actually called "My Addresses".

Let me make things even clearer.

The feature that others have pointed out is available is this one. Proton allows replying from the + suffix type of alias. That is, you can make an alias by using one of your addresses and suffixing it with a `+customalias`. But that exposes your address obviously. Anyone can simply remove the suffix and send you spam mail to your address. If that happens, you will be forced to delete or disable one of your limited quantity addresses.

For that reason, we would like the ability to reply to a message sent to a hide-my-email type of alias from the same alias.

This latter feature is certainly NOT available.

Edit: I was wrong. Read later comment.

I am not sure if everyone is talking about the same feature.

If I reply an email to one of those "extra alias" managed under "Identity and addresses" section of Proton Mail Settings, the composer defaults to replying from that address. We can generate up to 10 / 15 / 30 of these aliases.

But this does not work with "hide-my-email" @passinbox.com aliases generated from Proton Pass. We can generate an unlimited number of these aliases.

Multiple users commented that this feature is already available. However, I do not see any way to use it so any reply I make to an email received at an alias uses my primary email with no apparent way to change it. Please advise as to how I can instead use the same alias in the reply. Also, there should be a way to do this by default so the alias is used unless the user takes action to switch to primary.

As other comments said, this happens by default

When they send us an email to the alias we have created, that email is forwarded to the main email but we cannot answer using the alias we have created. It would be good for our security, to be able to respond with that Alias.

Yes, it would be good to email out using the alias without having received any email to the alias yet. E. g. for resolving problems or inquiry about status of procedure.

If it is important for Proton to block misuse of the alias function by using it as a second email address in the free plan, there could be an alternative form of limitational fencing:
Aliases could stay valid for a maximum of 6 months and be available for simple email reception and sending (including attachments). Expiration date should be displayed for each alias. I suppose most use cases regarding contact with potentially spamming companies would be covered inspite of such a limitation. And it could not serv as an additional email identity!

Just tested it. Works fine.
PLEASE close this issues as resolved / implemented!

Have the ability to send an email from a hide-my-email alias, I know you can reply using the alias but would like the ability to select it from a drop down menu (from the from section).

Apple is offering this for "Hide my Email" in Apple Mail, where you can select it as option from the "From" dropdown. It is a really great functionality, cause sometimes it is just necessary you contact services you registered for with an alias over an different mail like support@service.com, if you ran into problems with the service, and there you also don't want to show them your main Mail Adress

From the comments here it appears the feature already exists. However, when sending mails, it's very unclear if it is. Best would be to be able to select one of your aliases to reply with.

Additionally, the same would be useful for having a reply with your catch-all email. Example:
You can give theail address random-site@yourdomain.com on the fly. When catch-all is enabled and this email does not exist, it should automatically create this as a alias email (as you are the owner of the domain and it is set in proton, this shouldn't be that difficult as you can already do this manually anyways). So that when replying you can easily reply in the name of that email without sending mails from catchall@yourdomain.com

This feature is available. All you have to do is just normally reply from your email & it will automatically use your alias, even tho it says that it's using your email. That's something that could be improved because it's not clear enough.

If this feature is already available, will someone please tell me where or how to use it? What I see is that if someone sends me an E-mail using one of my aliases, if I hit the reply button, my account E-mail is in the 'From' field.

Giving out an alias is pretty useless if my actual account E-mail is exposed the first time I reply to an E-mail.

This feature is already available within the Proton offering.

You might need to create a reverse alias for the given alias first (I don't know if the are generated automatic) but if you reply, you reply to that reverse alias and hide your real mail address. You will notice if you look closely to the to: address.

So you might want to close this idea :)

Send mail from a hide my email alias