[ReadyPlayerNone]

I'd reconsider this change. Lots of people use email aliases to track which sites share their contact details with third parties. I see it all the time in signups for one of my sites - don't mind, of course, because I don't share their data. If you stop people doing this it might send the wrong message.

[ixwt]

This is a poor way to track who is selling your contact details. It's trivial to strip + aliases from a list.

[groestl]

But it's a good way to track who accidentially leaked your contact details.

[acct1771]

Unless the receiving party strips them as a matter of deliverability of their spam.