If you can provide the domain you are talking about we can take a look into it and narrow down the exact problem for possible resolutions.
run a report for that domain in
http://www.intodns.com
You may find some useful information for solving this problem.
Good Luck.
MailChimp is an excellent tool, our company have settled to mailchimp after trying
interspire email marketer.
We found interspire to be less flexible for marketing our flagship hosting trademark although it worked decently for our other trademarks.
Mailchimp is a lot more user friendly and has lot of options that we are using it to the fullest.