Cannot send e-mail

Sometimes when I am trying to send an e-mail from my iPad - it will not go through. It comes up with the message,

“A copy has been placed in your outbox. The recipient ' e-mail address' was rejected by the server because it does not allow relaying."

I have in the past successfully sent e-mails but often I do get the message above. The email message is indeed placed in the outbox on iPad but I would expect to be able to retrieve it on my MacBook pro but I can't.

Has anybody else encountered this problem?

Thank you for your help.