Assume that mutual TLS is configured between organization A and organization B in Microsoft Exchange Server 2019 or Exchange Server 2016. The user in organization A can't use Outlook on the web (formerly known as Outlook Web App) to reply the email message that's sent from anyone from organization B.
To fix this issue, install one of the following updates:
To work around this issue, you can use Outlook to reply the partner email messages in this situation.
Learn about the terminology that Microsoft uses to describe software updates.