When using Office 365 on Windows, there is no issue if the customer's email does not match with the Webex application. The behavior is not the same in macOS. If the Office 365 email does not match with the Webex application then the user cannot schedule meetings.
In order to answer this as-as, we will have to make a lot of assumptions. Best not to make assumptions.
Please provide detailed steps to reproduce. Where are you scheduling from the Outlook desktop client or OWA? What tool are you using to schedule the Webex Meeting? Which email address are you referring to? Does the "Webex application" refer to Webex app (formerly known as Webex Teams) or are you referring to the Webex Meetings Desktop App?
No response in over a year, so closing as Not likely to implement. We can always reopen if someone provides more details back on the issue brought up.