I'm really wanting to find a solid answer for this as well. We just switched our email over to Microsoft's BPOS (specifically the Online Exchange service) and it will allow you to use SMTP to send emails but it requires TLS encryption, no exceptions. I haven't found anything yet for the UC520 that will allow you to specifiy to use TLS (either through the CLI or GUI) in CME or CUE but if I find something definitive one way or the other I will respond here.
Thanks!
bax