Regarding "...I suspect that they have not implemented this because doing so completely eliminates end-to-end encryption,..." IF this is their rationale, I disagree with their logic. Like you, I have a professional association but those emails (and those I select reply-to to that address) are not privacy sensitive and I don't want those recipients to use my Proton address. I just want to use a single mailbox for both.

I too have an eternal email address from a professional society where I'm a lifetime member. I've used that address for nearly 30 years where it has pointed to various mail systems over time. I assumed that Proton would have this functionality as well, as it's just so basic, so I'm really shocked it does not. Now feel bad that I got a paid account before finding this out. I suspect that they have not implemented this because doing so completely eliminates end-to-end encryption, but the fact that they have not said as much does not speak well for them, and that could be dealt with through simply informing the users -- I want to be able to make an informed choice between confidentiality and availability.

Yes, why isn't it possible to add a reply-to field?
I even wish to be able to set it as a default under user preferences.

Will this come? I have an "eternal mail address" with a forwarder service, and I want people respond to that mail when I send mails from Proton. Anything in the pipeline?

Given how long this issue has remained open, there must be some real or imagined reason that Proton won't implement it. Is there perhaps a security issue? Proton, please at least comment.

Need this to include send from aliases
Example test.com alias to proton account
Send from j@test.com with a reply to of john@test.com

Hello, it's a simple feature : every mail provider has it.

II have two email "presences" that I need to maintain on outgoing emails, as an editor and as a licensed radio amateur. I need to be able to invoke a "reply-to" link for selected outgoing email messages. It works fine in gmail. I'd like to switch to proton mail for everything but I cannot without a selective "reply-to" choice.

If you send mail via Proton Mail Bridge with a Reply-To field, it gets stripped off. This suggests this is a deliberate lock-in policy. Not good.

We need it please !
Especially when you can't use imap with a Proton account

Just signed up for a Proton mail account and very disappointed not to have this feature.
This has been reported so often that Im surprised that Proton refuses to offer it.
I will continue to search for another alternative to Gmail that does. Good thing I did not already choose the paid Proton service.

The fact that this is not implemented is shameful.

Special handling for Reply-To is not critical, but being able to add standard email headers definitely is. There's no good reason not to have an escape hatch.

Here too.

Ping! Still mission critical for me.

Absolutely essential!

How is this still not a thing?

Must have when your main domain is hosted elsewhere (and has to remain elsewhere). Mail forwarding and reply-to are jointly the solution to my situation.

This is a must-have for me and is supported on practically all other platforms. Have been waiting for a comment from Proton for a year now. I'm going to move away from paid version to some other platform at the end of this billing cycle.

The reply-to field allows the sender to specify a preferred email address for reply mail. This feature is present in most modern email services and is a standard feature of email.