Microsoft have changed their interface for Office 365 admin and the Mimecast details have not been updated
Firstly logon to the Office 365 EAC by typing in https://podXXXX
.outlook.com/ecp/ where XXXX is your OWA pod number for Office 365.
Click on mailflow and then Connectors from the mid top menu.
- Add a new Outbound connector and give it a name
- Tick Connector type Partner
- Use Opportunistic TLS
- Click on Route mail through smart hosts
- Enter the smart hosts according to the Mimecast MTA documentation here
- Leave the scope unchecked
- Add the outbound domains you want tho apply to this connector
That should be it for outbound....Test an external email straight away to check all is well.
For more info see