Microsoft Send

Messages to and from the Send app do not utilize subjects lines.


The message goes through the same outbound mail servers as if I sent it from Outlook or OWA, so emails pass SPF.

If you are signing with DKIM, the emails will be signed and authenticated.

(Office365 will sign emails with DKIM soon, it’s on the Office RoadMap.)
For an email to show up within the Send app, the subject contains #Send.
Microsoft has taken a unique approach to building a messaging app that utilizes existing SMTP infrastructure. If you’re sending to a tech savvy list, take a look at your logs to see how many recipients are using Microsoft Send and consider reaching out to them specifically using #Send.