Hi Edith36,
Welcome to the Support Communities!
Are you getting this error message sending an email to one particular contact, or are all of your Sent emails not going through?
The article below will walk you through the troubleshooting steps for email on your iPhone:
iOS: Unable to send or receive email
http://support.apple.com/kb/TS3899
I hope this information helps ....
Have a great day!
- Judy