There are black lists of IP addresses that you don't control. Let that sink in and do your research as it's pretty well done in discussions and some folk flame anyone that dares bring it up. The web mail is likely not in the black list.

As to how to resolve you have to trace the email and see which servers the email traverses (sorry if I shortchange you on how email travels or works) then if you are lucky see where it drops out. Hopefully some bounce message came back with this detail otherwise you get to ask them all.

Now there is something else. You write you changed the IP address but didn't reveal a small detail. If you were using something like pop3.email.com the IP change will take up to a few weeks for DNS to propagate. Again I'm going to shortchange you on how or why this is. AGAIN, you didn't say you used a friendly name for the address you stated that you used the IP address.
Bob