Unable to send messages iPhone

I have the iPhone 4. I can receive and send email through yahoo, but I have another account through Zimbra which is my work email. I can receive mail through Zimbra, but I cannot forward messages to my husbands Droid X nor can I send any messages out from the Zimbra account through my iPhone. The error message is that the server does not allow relaying. What does this mean and how can I fix it? (My husband hasnt had any problems sending and receiving mail with his Droid X through Zimbra) Any help would be greatly appreciated! Thanks!
Windows 7