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:
For Exchange Server 2019, install the Cumulative Update 2 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019.
To work around this issue, you can use Outlook to reply the partner email messages in this situation.