MD5 encryption was phased out for email authentication. You need to change the authentication method in your email programs.
General Info:
http://www.rackspace.com/knowledge_cent ... g-cram-md5
Fix for Mac Mail:
http://www.rackspace.com/knowledge_cent ... n-mac-mail
Fix for iPhone:
http://www.rackspace.com/knowledge_cent ... ios-device
Fix for Thunderbird:
http://www.rackspace.com/knowledge_cent ... hunderbird
