I should've been more precise; em dashes are just a piece of the puzzle, however, the smart tools you reference are normally programmed to convert dashes to em-dashes under the following condition:

Text > space > hyphen > space > text

Unless you go back afterwards and delete the spaces between words, the AI text is going to stand out due to its tendencies to use em-dashes in place of semicolons without spaces—like this.

It does suck that we need to take a reactive approach to dealing with the perception our emails might not be written by us, or...

Take it back by using them anyway, and insisting our messages are legit.

Though, unless AI stops generating text with far less em-dashes, I think this is an uphill battle which isn't worth our stubborn insistence. though, that really depends on whether it's important for our messages to seem to be written by us.

For me, where I work, people have become very judgemental (though not everyone knows) when they receive an email heavy with em-dashes.