Just a quick heads up, as this caught me out today. I am not even sure if this is the case in SP1 for exchange and I have no time to check, but it is in Exchange SP2.
If you are running GFI MailEssentials 11 on your Exchange server, and are using the List Server element of this great application, make sure to uncheck this highlighted box! If you don’t, your lists will not work from external addresses. (MailEssentials will still filter using the directory harvesting option).