Would love this feature, as I have multiple catch-all custom domains set up and use forwarding to my wife and me. It would be great to be able to send from these wildcard addresses so the sender sees the address.

Proton Pass allows for aliasing using custom domains, which proved that this is technically possible. See: https://proton.me/support/pass-use-custom-domain

Critical! This missing feature is the only reason I have not yet migrated my primary custom domain to Proton.

Currently I maintain a separate outbound SMTP server and use Thunderbird (with relevant SPF records in DNS) just for this specific use case. I would prefer to NOT have to operate such a service and just use Proton Mail alone for this. If another service comes along that offers this competitively, I will cease using Proton Mail.

I have since moved to another provider to gain this basic functionality.

I use unique email addresses at my custom domain every time I have to give out my address so I can tell, if my data is breached, where the source of the breach was. Right now, to reply, I have to add an address to my account, send the email, then delete the address again. It's not unmanageable but I might end up moving to another provider when my renewal comes up in November, as plenty of them offer this feature.

This is an incredibly important feature that I use as well. I also use it for several mailing lists that I'm subscribed to. This is something that I routinely use in Thunderbird.

I just subscribed to Proton Mail and I hit this issue. This would be a big quality of live improvement.

I would agree this is a very important feature. I would assume that allowing unlimited (large)number of addressees per domain might be challenging due to encryption (as each address has a set of associated keys). This solution, however, could work without the keys.

This is a very important feature, I just subscribed and started with Proton. This will be a deal breaker if I will continue or not.

There are dublicate to this request. If they could be bundled, this feature would move up in the wanted features list :-)
https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/41522041-reply-from-catch-all-address
https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/45429793-allow-custom-domain-name-with-catchall-to-also-sen

There are dublicate to this request. If they could be bundled, this feature would move up in the wanted features list :-)
https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/41522041-reply-from-catch-all-address
https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/36214159-send-from-any-address-of-custom-domains-if-catch-a

what @thomasedward said.
Critical for me as well to migrate one of my domains to proton.

People may want to combine their votes

Duplicate or similar to: https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/45429793-allow-custom-domain-name-with-catchall-to-also-sen

This would be amazing if it was possible. Especially if it just did it automatically, if there was a flag to turn on the option. "Always reply with envelope recipient address".

This is CRITICAL.

I have a custom domain specifically so I can give each merchant I'm dealing with a unique address. For example let's say I was doing business with McDonalds, I'd give them the email address mcdonalds@[mydomain].com, then the catch-all for [mydomain] would receive it. But now if I want to send an email in reply, it will come from info@[mydomain].com. I need to be able to set the "from" field manually so that it instead looks like the email was sent from mcdonalds@[mydomain].com.

I do not want to have to set up a separate address for every single merchant - that's totally unworkable.

Yes please! This can be confusing for the sender who gets replied to from a different address than what they sent to.

This is VITAL! I cannot use Proton Mail for all my email purposes until it has the ability like Mozilla's Thunderbird email client for my email messages and replies that I send to ORIGINATE FROM whatever domain and address I choose (for domains I forward to Proton Mail and have appropriate DNS SFP records set to permit mail origination from Proton's ervers--or for replies to messages I have received in my Proton Mail mailbox because Proton is set to accept email to ANY or ALL addresses for that domain). As it is now, I rely on a different mail server for handling this. It would be nice to move all my email to Proton, but until I can set identities for messages and responses for domains I own, I cannot use Proton Mail like I wish that I could.

It's best to leave the catchall turned off. With catchall turned on, if someone sends an e-mail to a non-existent address, it looks to them like it went through to the intended recipient. Leave it turned off so the sender gets back a response telling them that they made a mistake with the address.

Second this, ability to be able to send from any catchall from custom domain or ability to add unlimited amount of custom domain usernames seems vital.