After the Microsoft Azure Active Directory Sync Tool runs to sync your on-premises Active Directory environment to Azure Active Directory, a user’s Office 365 email address unexpectedly contains an underscore character "_".
This occurs if the user account in your on-premises environment has a proxyaddresses attribute or mail attribute that contains one or more of the following special characters:
single quotation mark
In this scenario, after directory synchronization is run, the special character is replaced by an underscore character. Therefore, the user’s Office 365 email address contains an underscore character instead of the special character.
Remove the special characters from the proxyaddresses attribute of the user account in the on-premises environment.
The characters that are listed in the Cause section are treated as special characters in Office 365. Use of any of these characters may cause issues such as service disruption.