Port 993 is for the incoming mail not SMTP.
You're running 10.6.8, yet you post to the Mountain Lion section.
There is currently an issue with the servers: http://www.apple.com/support/systemstatus/
Email cofiguration settings for iCloud using Snow Leopard: http://www.wilmut.webspace.virginmedia.com/notes/icloudmail.html
Could it be that I need to upgrade to Mountain Lion.
Delete the mail account, making sure to delete the SMTP server settings too, and start over again:
Pay particular attention to the part about selecting IMAP over the default suggestion.
There was an issue with servers this morning, but it now appears to be okay. At least for me anyway.