There is a Smartermail server on a customer network, with 1 domain configured and that domain has 1 alias domain.  DKIM signing is configured and working.

There are several appliances on that network which send administrative alert emails, relayed through the SM server, and the FROM address is the alias domain, such as array1@alias.domain.com, tempsensor1@alias.domain.com and so forth.

The entire LAN subnet is whitelisted in Smartermail for SMTP relay.  We don't have mailboxes for each and every appliance, because mail never gets sent TO them.

The problem is the recipient of these administrative alert emails is hosted on Exchange 365, and the messages are being tagged as "unverified sender".  We checked the headers and the messages are NOT being signed by DKIM.  

Does DKIM not sign when the FROM address is the alias domain?  Or is the root cause because messages are (whitelist) relayed without each device authenticating before sending the message?  Other?