Here are some SMTP links I found that could be helpful.
SMTP Queue Backup Occurs Due to Delivery Threads Attempting to Connect to Unavailable Hosts
http://support.microsoft.com/kb/264891
SMTP Service Design
http://technet.microsoft.com/en-us/library/b8aafeb4-8883-482b-8cf8-77b3975336b3.aspx
http://technet.microsoft.com/en-us/library/b8aafeb4-8883-482b-8cf8-77b3975336b3.aspx
Great resource for mail related things
http://www.msexchange.org
Google is your friend
http://www.google.com/search?hl=en&q=site%3Asupport.microsoft.com+iis+smtp
DNSStuff.com – This is the Google of DNS troubleshooting sites. The best hands down!
http://dnsstuff.com