It works for many mail servers, including Gmail.

Also any Google Apps servers. Seems like a lot of extra work to look up a domain's MX records to figure out if it's safe to strip anything after + from their email address, which will only serve to annoy your users anyway.

which others? doesn't do it on sendmail or postfix

It does work on gmail, but is definitely not gmail specific. Used to do that trick on my college email account.