Comment 9 for bug 702097

(In reply to Wayne Mery (:wsmwk) from comment #4)
> I don't think we'd bother with such an obscure issue. But I could be wrong

Yeah, [lacking any other reports](https://bugzilla.mozilla.org/buglist.cgi?quicksearch=%3Athun%2Cmailn%20undisclosed-recipients&list_id=16122726), this doesn't seem to be a big issue - but I'd feel better if we could test this against a recent version of Outlook before closing the bug.

Both `To: undisclosed-recipients: ;` or localized equivalents like `To: Verborgene_Empfaenger: ;` are valid syntax, `named groups` per [rfc5322](https://datatracker.ietf.org/doc/html/rfc5322#section-3.4), last updated Oct. 2008:

> group = display-name ":" [group-list] ";" [CFWS]

Thunderbird's usage is explicitly mentioned in the specs (see bold format added by me):

> When it is desirable to treat several mailboxes as a single unit (i.e., in a distribution list), the group construct can be used. The group construct allows the sender to indicate a named group of recipients. This is done by giving a display name for the group, followed by a colon, followed by a comma-separated list of any number of mailboxes (including zero and one), and ending with a semicolon. Because the list of mailboxes can be empty, **using the group construct is also a simple way to communicate to recipients that the message was sent to one or more named sets of recipients, without actually providing the individual mailbox address for any of those recipients.**

So Thunderbird isn't doing anything wrong technically, and as reported, this would be a bug in Outlook.

That said, imo it's quite easy for the user to send to a group of `BCC` recipients without adding a `To` recipient, iow it's quite easy to send out messages having `To: undisclosed-recipients: ;` syntax. *If* such messages are still breaking in Outlook (which I doubt), we should probably explore better options than letting plain vanilla messages break in Outlook without much benefit.