Deliver log with "msgid=?" when email come from Exchange/Office 365

Hi,

I noticed that all emails that come from Exchange/Office 365 have the message-id, in deliver log, reported as

msgid=? <DBBPRDD0@DBBPR023.eurprd08.prod.outlook.com>

instead of expected

msgid=<DBBPRDD0@DBBPR023.eurprd08.prod.outlook.com>

My deliver_log format is:

deliver_log_format = deliverytime=%{delivery_time}, msgid=%m, sender=%e, from=%f, subject="%s": %$

The behaviour is the same on Dovecot 2.2 and 2.3, and only dovecot have the "?", we have a mail filter before dovecot and the message-id is registred fine, without the ? in front.

Is an issue of my setup or a bug in dovecot? Thanks

-- Alessio Cecchi Postmaster @ http://www.qboxmail.it https://www.linkedin.com/in/alessice