Relevant Product: Exclaimer Cloud - Signatures for Office 365
After running the initial connector setup wizard for Exclaimer Cloud, the user’s email addresses have changed to use the .onmicrosoft.com domain associated with your Office 365 subscription.
Microsoft has confirmed that this occurs when the data is synchronized from an on-premises Active Directory to Office 365/Azure AD and the proxyAddresses attribute for users are not populated in the on-premises Active Directory.
The proxyAddresses need to be populated with the relevant SMTP addresses for the users.
The following PowerShell script has been provided by Microsoft to set the proxy address for a list of users stored in a CSV file: