2822 To: header (was Re: [ietf-dkim] Mailing lists as 2822-Sender)

Hector Santos wrote:

> in the same vain many SMTP receivers will fill in the missing 822
> required fields.
>    FROM:  <<-- x821.MailFrom
>    To:    <<-- x821.RcptTo
The latter should never be done.  If there's a missing To: header,
an SMTP receiver should synthesize:

     To: undisclosed-recipients;

Otherwise, you risk exposing Bcc: addresses.

(I suppose it's safe for the MDA to add a To: header upon final delivery
to one and only one recipient.)

Regards,

David.