We run an internal SMTP relay server, on 32-bit standard edition Windows 2003 Server, via IIS6 virtual SMTP server. We need to turn on TLS for some but not all external recipients.
I've read that when you enable TLS on the SMTP server bundled with IIS6 the SMTP server will require TLS support on ALL remote hosts it tries to send email to.
Is there any way to enable 'Opportunistic TLS' on this or Windows Server 2008? That is to say where our server will only send mail using TLS if the remote SMTP server advertises it is capable of accepting it, and will send all other mail normally, to those who don't accept TLS email?