JavaMail Exception: javax.net.ssl.SSLKeyException: RSA premaster secret error

I get this error when I try to connect through Javamail to an IMAP server using STARTTL开发者_如何学JAVAS. I thought it might have been the (pointless) US export restrictions on the JCE, so I double checked that I'd replaced them with the unlimited strength files. Same problem.

Can't find anything particularly relevant on google or here (in fact the stackoverflow search seems to be throwing server errors today).

Anyone else seen this before?