Stick to 5321 for now.

[gennyble]

There's a draft for an updated version of 5321 that changes some behavior- found here (text; status page). Through discussion, we've decided to stick to the current standard and, if the draft is accepted and given a number, to change behavior only then.

This does mean that we have to accept an empty Quoted-string, however. That means that this mailbox, ""@example.com is valid. The draft specifies that a Quoted-string must have at least one character inside of it.