Comment 2 for bug 702097

User-Agent: Mozilla/5.0 (X11; U; Linux i686; de; rv:1.9.2.13) Gecko/20101206 Ubuntu/10.04 (lucid) Firefox/3.6.13
Build Identifier: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.13) Gecko/20101208 Thunderbird/3.1.7

Mails sent from Thunderbird which merely contain BCC entries generate a TO: header in the de_AT.UTF8 locale like:

To: Verborgene_Empfaenger: ;

This header is successfully parsed by:
Microsoft Outlook Express 6.00.2900.3664
for example Outlook version:
Microsoft Office Outlook, Build 11.0.5510
does not decode these mails correctly.

The the raw MIME format including all the headers are presented.
I'm not sure whether the To header must contain a valid address,

Evolution warns about an empty To: header invoking an Apparently-To: header on some systems revealing the BCC recipients. Maybe a warning with the suggestion to add the sender as the To: recipient would satisfy both cases.

Reproducible: Always

Steps to Reproduce:
1. compose a mail leaving the To: field empty but adding an Outlook Build 11.0.5510 (Haven't had any feedback from to many other Outlook versions)
2. send the mail
3. receive the mail in Outlook Build 11.0.5510
Actual Results:
The mail is sent without a warning. A localized recipient is generated:
To: Verborgene_Empfaenger: ;
Outlook renders the raw MIME message including the headers as the mail contents.

Expected Results:
A warning about missing To: field would be great with a suggestion to enter the sender a value which can simply be confirmed. If the header:
To: undisclosed recipients: ;
is legal, then that message shouldn't be localized.
Outlook should render the mail.

Not sure if this is a real bug on the side of Thunderbird or if Outlook is being buggy but it the end user impression is that Thunderbird is sending buggy mails.