To clarify the SMTP RFC (RFC 5321) says that a valid e-mail address is defined in RFC 5322 - Internet Message Format.

RFC 5322 does say that a `+` is a valid character in the local part of an e-mail address.

The `+` character being used for address aliasing is, as far as I can tell, not mentioned in RFC 5321 or RFC 5322

It's apparently a thing now:

https://tools.ietf.org/html/rfc5233

It's not.

See the sibling comment: https://news.ycombinator.com/item?id=20109564

RFCs don't define any meaning to '+' as described in this thread, except that local part of the address should be interpreted locally (usually by MDA), and preserved unmodified during message transfer.

There's no + aliasing in the specs. There's no interpretation defined for local part of email address.

Oh, yes. I meant "+" is an acceptable character to use in the local part of an email address, as per the RFC. I see now where the misunderstanding lies.