SMTPUTF8 issue 3: unintentional "punycode" for localparts

[arnt]

Hi,

I've seen three different software developers send mail from:<xn--gr-zia@xn--gr-zia.org>, not understanding that while punycode can be used for domains, the localpart has to be UTF8.

xn-- localparts are legal, and John Levine has such an address (xn--something@outlook.com), so this isn't necessarily an error, but very likely to be. It would be good if aboutmy.email could point it out if one or more address fields contains xn-- in the localpart.