Good point. But I doubt any significant share of users will consider doing it. Most of them are of course unaware that it’s even possible.

Perhaps someone here has some statistics to share? It would be interesting to know how common this trick is :)

[Edit: Better wording]

Actually yes, I remember now that's why I use - in my Postfix instead.

Postfix allows you to choose any character as a separator.

Yep. But it can be circumvented if one’s mail server use another delimiter. E.g. ”.” will hardly be filtered.

For things I care about the domains are owned by me, for things I consider spammy I have a round robin of afraid.org aliases which are for all intents and purposes disposable. They are very effective for this sort of thing, but you would be wise to do your research into the history of the domain you're piggybacking to be sure it's not going to disappear.