One of the problems you can have is ISP related. For example one the US's largest ISP's (Cablevision - optonline.net will bounce any e-mail received from THG. According to them, one of the hops that the e-mails get routed through is in the US Database of Known Spammers. Since they system blocks any traffic from anyone on that list, I can not receive e-mails from THG on my regular ISP account. I have to use a web site based account and even though such is provided by the same ISP, this works.
So if the anyone on the hop list when you run a tracert is in this database, e-mail from THG is not going to happen if your ISP adopts this system of "protection".