I use SimpleLogin and recently replied to a sent message, which embedded my core email address in the body of the message making it visible to all recipients

In the same way there are alerts inside SimpleLogin to say, for example, that you cannot reply to yourself from an alias, it would be very helpful for there to be a real time scan to prevent your core email address being visible inside an email. This could either be a toggle in global settings or in the reply panel itself to enable exclusion of those recipients who have Proton mail addresses

Remembering to go through a whole email looking for a core email address isn't going to happen, so some automation would make sense.