I have a requirement in my project where I have to secure the communication between my client application (running on BizTalk Server Send port configured with Office 365 Email adaptor) and Microsoft Office 365 Server. This communication is to be secured by TLS 1.2 and a certificate is provided by MS O365 server during Server hello (handshake process) to later securely exchange the symmetric key generated at client side.
Do I need to install another certificate at client side for either generating this symmetric key or for any other purpose to achieve a better secured communication between the client and server?
No, you don't require another certificates client side (BizTalk) to establish a TLS 1.2 connection.
The BizTalk Server needs to trust the certificate however, but as this is one issues by Microsoft, it should be trusted.