Hello Binesh! I reviewed the headers you provided and it looks like this might be due to the connection to one or more of your URIBL lists not returning quickly enough. Can you try setting your primary/secondary DNS server addresses to Google under Settings>General?
Google DNS 1: 8.8.8.8
Google DNS 2: 8.8.4.4
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com