What about + signs? That's the trick I use.
Now odds are, someone is going to respond with the one service where the + becomes significant..
It seems to only talk about the receiving server and gives no way for the sending server to do anything useful as it doesn't specify the character to use if the receiver implements it at all.
I think this is good, I would like to not have senders try to guess anything about the structure of my email, but it makes me wonder what the point of having the RFC at all is.
Why? I'm not sure it has ever been a standard and it's certainly not a default implementation for most providers?
Adoption is a mixed bag.
But at the end of the day doing this kind of comparison probably raises more questions than it solves.
The + trick arose later, as a feature of certain mail recipient programs like Sendmail and Postfix, and most of these programs allow you to change from + to any character you like.
All RFC5233 does is specify how sieve (a protocol for specifying server-side email filters) can describe filtering on subaddresses. It does not confer any actual interpretation on email addresses, especially since many email providers don't actually use sieve for their email filters.