Although '+' is an acceptable symbol, there's actually nothing in the RFCs that mandates sub-addressing behaviour. The closest you'll get is RFC5233 that defin

Although '+' is an acceptable symbol, there's actually nothing in the RFCs that mandates sub-addressing behaviour. The closest you'll get is RFC5233 that defines it for the Sieve filtering language. So although very common, sub-address behaviour is not a standards requirement and should not be presumed. By "heavy marketing" I presume you mean spammers. If not, be warned, sub-address stripping will mean nondelivery in some cases and could trigger blacklisting via honeypots.