This happens because Microsoft hasn't yet authorized your new tenant's outbound IP, and it's most likely to show up when you're onboarding a newly added tenant into your ExSign environment, so it's worth knowing about, even if you haven't hit it yet. This requires Microsoft Support to actually authorize the IP on their end. It isn't something you can fix immediately yourself, budget for a wait, not an instant resolution. 

Requesting Authorization 

1. Navigate toOffice.com and sign in with your Microsoft 365 admin credentials.  
2. Click the Admin tile to open the Microsoft 365 Admin Center.  
3. In the left-hand navigation, go to Support >> Help and Support.  
4. Click New Service Request (or Contact Support, depending on your interface) and fill in: 
   
   

   Field	What to Enter
   Issue Type	Email and Calendars, or the closest related category
   Subject	Something like: "Need to Authorize New Tenant IP"
   Description	See the template below
   Tenant Details	Include your tenant information to help Microsoft process the request faster

   
   
5. Review the request for accuracy, then click Submit (or Send).

After You Submit 

Watch your email for a response from Microsoft Support. Once they confirm the IP has been authorized, send a test email from the affected tenant to confirm the error is actually resolved before considering this done; don't just assume it's fixed once you get a confirmation email. 

If it's been a while with no response, or the error persists after authorization, follow up directly with Microsoft Support rather than starting a new request from scratch.