+ is not a unique convention. I have dynamic rewrite sets so that all mails with specific prefixe s go to my mailbox:

<my initials>-site-<companyname>@<my domain> go to my personal mailbox

<my partner's initials>-app-<appname>@<her domain> go to my partner's mailbox

<daughter initials>-account-<entity name>@<my domain> go to my daughter's account

Sure you could in theory set up the server side verification mecanism for these pattern too. I am just stating that the +suffix stuff is not the only way used.

They cannot without cooperation of the mail provider as mentioned earlier.