Update 5/20/2026: Added a note about Change Optics Report in EAC. To help shed more light on the Direct Send feature, we published a newer post called Direct Send vs sending directly to an Exchange O...
Updated May 20, 2026
Version 16.0
Blog Post
Carolyn_Liu
Microsoft
Microsoft1. Does "we currently have an inbound connector for Mimecast in Exchange Online" mean a Partner Inbound connector? If so, unless you check the Reject email messages if they aren't sent from within this IP address range checkbox in your Partner connector (step 9 in Set up a connector to apply security restrictions to mail sent from your partner organization to Microsoft 365 or Office 365 | Microsoft Learn), the connector was really enforcing the inbound emails to your organization. 2. Is your domain's MX pointing to Mimecast? If so I would discourage you to use IP addresses unless you have dedicated IP addresses from Mimecast, otherwise you have to monitor all Mimecast IP addresses, if they add a new one, your partner connector enforcement will end up rejecting emails from those new IPs. You probably should consider using mimecast certificate domain instead.
Yes, we do have the dedicated IPs from Mimecast for our region. Also, can you please confirm on Calendar and meeting invites? Plus do you recommend enabling reject direct send as well or will that cause issues with Microsoft notifications and out of office replies?
- Carolyn_Liu
Microsoft notification emails (SharePoint and Teams) are sent to your recipient domains. If the recipient domain is xxx.onmicrosoft.com, the MX would point to M365 and won;t come from Mimecast. Calendar and meeting invite should be OK, I assume. You can check the headers for those emails to see the routing path. In short, emails/notification/invite from Microsoft are following the MX record.