Your business needs all of its email to reach customers, but it is getting harder and harder to assure that delivery will occur. There are many factors which may be beyond your control and will have to be addressed by your server administrator. Often times the larger companies like Comcast have in place rules that smaller companies can not implement for a number of reasons. One being how they deal with incoming spam messages.
What you can control is helping your customers by having them whitelist your email address. This is sometimes called adding an email to an Approved or Senderlist. Click here for specific information on setting up a Whitelist/Approved/Sender List