We've actually addressed this several times on a couple of different threads.
90% of the time it is not your customer but their email provider. The worst of them are AOL, MSN and Yahoo where they advertise to their clients how well they block out spam. The problem is they are blocking out literally thousands of services such as Shaw, Rogers, Sympatico, Cogeco and on and on.
As soon as lets say Rogers gets things cleared up, wham AOL makes a change and Rogers is blocked again. That change may be the result of a Rogers account spamming an AOL account so all Rogers gets blocked.
These are bandaid solutions to the big problem of spamming.
So far the best method to get your emails through is to set up several email accounts with different providers such as hotmail etc.
Malcolm
