I use DuckDuckGo Email Protection for my accounts in a lot of places, one of which happened to be a credit card company. I needed to file a support ticket for it and, rightfully so, they require the sender to be the original email that I signed up with. I was expecting this to be a bit of a nightmare but thankfully DuckDuckGo already thought of this and have a rather simple way to do this.
If your alias is john.doe@duck.com and you wish to send an email to jane.doe@example.com, you can do so by sending an email from your forwarding address to jane.doe_at_example.com_john.doe@duck.com. What we did here is replace the @ in the receiver address with _at_ and then append our alias, again separated with a _. This lets this continue being a valid email while being unambiguous for DuckDuckGo’s routing infrastructure. I tried it out and immediately got the confirmation that my support ticket had been accepted! After the initial hurdle replying normally will continue to work.