Test your mail server:

http://mnathani.com/blog/2012/09/28/perform-tests-on-your-mail-server/

Test your name server:

http://mnathani.com/blog/2012/09/25/perform-tests-on-your-dns-server/

Test your IPv6 connectivity:

http://mnathani.com/blog/2012/09/28/test-your-ipv6-connectivity/