In the Office 365 Admin Center, under Users > Active Users, you can see a user called imserviceuser, with no licenses assigned. Under display name, you will see "User account used for integration with third-party services", and the Username will be imserviceuser@yourcompany.onmicrosoft.com. Status will say Unlicensed.

This user is used for syncing Active Directory objects between Office 365 and Intermedia. It cannot be renamed, hidden, or deleted.

imserviceuser in the O365 Admin Control Panel