ssmtp and message ID

[Matt]

Hi all,

I'm using ssmtp instead of sendmail as the mailer on my FreeBSD
10.1-RELEASE box. The SMTP relay I point it to is my Synology NAS
running the mail server application. What I've noticed is that all mails
sent by ssmtp appear as having no Message ID in the log on the Synology.
I use the Synology with several other email clients, eg android phone,
ipad, kmail, alpine, thunderbird and all of them show a valid Message ID.

Do you have any idea why the Message ID isn't shown for emails sent by
ssmtp?

Thanks for any suggestions.
--  Matt