Have you used an outside mailing application such as Constant Contact? Many people don't realize that an IP range can get a bad name and therefore you get blocked by Comcast or any other major ISP.

Using something like Constant Contact can help get your emails through as companies like Comcast have more faith in the systems with this type of list management.

Comcast has a long history and there isn't much you can do except contact them and try to work it out.

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