This is how I configured Aerc, my email client, to work with Protonmail Bridge after I setup my custom domain with a catch-all address:

In accounts.conf

[protonmail]
from = Spencer Heywood <spencer@heywoodlh.io>
aliases = Spencer Heywood <*@protonmail.com>,Spencer Heywood <*@pm.me>
<snip>

This configuration will assume I want to use spencer@heywoodlh.io as my reply address (the default address for this custom domain) unless the recipient of the email is my protonmail.com email address or my additional pm.me address. Alternatively, you could provide the full address of your Protonmail addresses in the aliases section.

This is required because Protonmail has an irritating limitation on sending addresses.

BONUS #

Aerc configuration at the time of writing: home/base.nix#programs.aerc

My Protonmail Bridge Kubernetes deployment at the time of writing: protonmail-bridge.yaml