Running an Exchange 2003 setup and have been asked by one of our clients to enable TLS. I understand the concept and reasoning for this. However, my question is what consequences making this change to our server will have on all other clients that don't have TLS enabled on client side? Or is this a non issue? Basically will the mail still flow in cases where TLS isn't activated? Any insight would be appreciated.
TLS will only be enabled on the connection if both sides support it. Otherwise, in most cases, the message will be delivered in plain text, without encryption. However, there are some MTA's that are configured to abort the connection if the other side does not support TLS, or if the certificate check fails.