When an outlook message is attached to another outlook message, the message
is attached with the following header:

------_=_NextPart_003_01C4376E.933B71DB
Content-Type: message/rfc822
Content-Transfer-Encoding: 7bit
content-class: urn:content-classes:message
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="----_=_NextPart_002_01C43210.6E5E10F8"
X-MimeOLE: Produced By Microsoft Exchange V6.0.6487.1

Anton Mazkovoi said this:
Thanks for the info. As I have feared, the message is not attached with "Disposition" of "attachment", so JIRA will ignore it.