Why Microsoft Outlook should not be used to send email to a mailing list

I’m very supprised to get learned that Microsoft Outlook doesn’t respect RFC2822 about the using of “Message-ID:”, “In-Reply-To:” and “References:”.

3.6.4. Identification fields Though optional, every message SHOULD have a “Message-ID:” field. Furthermore, reply messages SHOULD have “In-Reply-To:” and “References:” fields as appropriate, as described below.

Most of the archivers for mailing list use these fields to show emails in threads, such that you can follow a thread easily. However, outlook does not use “In-Reply-To:” nor “References:” when you reply an email. So the email sent by outlook always start a new thread, even it’s a reply to a previous email in the mailing list.

So please, please don’t use outlook to send an email to a mailing list. Several better alternatives exist for you to choose: Mozilla Thunderbird, Evolution, and Gmail.