The recipient's server is rejecting your message. Have you tried sending it via both your wireless network and mobile data? On another wireless network?

Thanks Ed. I have tried it both through wireless & through my mobile network. Two different messages. I forwarded the message to my gmail account and then sent it to the person intended. Not sure what will solve this in future but thank you for your suggestion.

Do you have remote images in the message, e.g. in a signature? Does the message contain several links? These are the kinds of things that might trigger spam filters. Can you send a plain text message with no signature to this recipient?