Sometimes when e-mails are sent, they get stopped at the receiving e-mail server because the e-mail may not be recognized or thought to be possible spam or junk. In order to prevent e-mails from being stopped from delivery, white listing an e-mail address may be necessary. 

White listing basically tells the e-mail server that an e-mail address is safe for delivery.

Read this article titled "White Listing Steps" to view the attachment and find the appropriate e-mail provider. This information comes courtesy of www.whatcounts.com.