Had to configure a send port today to post an XML message to a website secured over HTTPS. The send was failing, with the following being written to the even log:
"The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel."
This got me thinking that we must be missing the public key for the site but I was able to browse to it from Internet Explorer and could confirm that the key could be found in the expected certificates stores on the client (my) server.
In the end, the problem turned out to be caused by the fact that 'Enable chunked encoding' was enabled in the 'General' tab of the HTTP transport properties page for the send port.