Could you please tell which RFC says this? According to RFC5321/5322 + can be used inside a local part but has no special meaning. Because a receiving MTA can i

Could you please tell which RFC says this? According to RFC5321/5322 + can be used inside a local part but has no special meaning. Because a receiving MTA can interpret a local-part as it wants [1] some decided to treat a part after + as an extension, but it is not universal. [1] RFC5321 "the local-part MUST be interpreted and assigned semantics only by the host specified in the domain part of the address"