We have a customer running SBS2008 and Exchange2007. Messages sent from one of their vendors are being bounced. We have added the I.P. address to the IP Allow List but that did not allow the messages to be delivered. The message size is small, 26kb. The content is confirmation of a proposal. The message is being sent in HTML format. The I.P. range of the sender is 207.x.x.x. The vendor also sends regular correspondence with an I.P. range of 74.x.x.x. These messages are not bounced.
Any suggestions on other items to check will be appreciated. Are there issues outside Exchange that can bounce a message?