Windows – The server does not support a SSL connection

http://getsatisfaction.com/mozilla_messaging/topics/thunderbird_2_to_3_0_upgrade_unable_to_authenticate_to_smtp_server_uncheck_use_name_and_password_for_that_server

Up till now the hackers have succeeded in breaking each and every protection. So I would say that the Windows Live message is a positive indication, but that one should still use normal caution.

As regarding what you should add to your own emails, see The Sender ID Framework :

Sender ID seeks to verify that every e-mail message originates from the Internet domain from which it claims to have been sent. This is accomplished by checking the address of the server that sent the mail against a registered list of servers that the domain owner has authorized to send e-mail. This verification is automatically performed by the Internet service provider (ISP) or the recipient's mail server before the e-mail message is delivered.

To use SIDF, e-mail senders and domain owners must publish or declare all of the Internet Protocol (IP) addresses used by their outbound e-mail servers, or the IPs authorized to send e-mail on their behalf, in the Domain Name System (DNS). These IPs are included in a Sender Policy Framework (SPF) text file.

In the best case, all this is already taken care of by your ISP.
But it is best to make sure.

In any case, the safety this method gives is based on the DNS system, which we know can be hacked; on the ISP, which we know may be rogue; and on the contents of the email, which can be forged. So Sender ID is surely hackable.