First off, you need to increase your diagnostic logging for the IMC, at a minimum you want message transfer and smtp interface at maximum. This will track your connections and messages inbound/outbound in the Applications Log.
That is one way to track problems. The other for this is to try and traceroute to the mail server in question that can not send to you. There may be network congestion, some of the routers do not have the proper routes. I have personally seen this, where I could not traceroute to a server from our main internet point, but from another point across the country I could traceroute there.
The diagnostic logging would be the best way to track problems of this nature, or contacting the site in question, to see ifthey know of why mail isn't getting to your site.
Hope this helps.