Receiving but unable to send iPhone

Hi all,
I have an iPhone OS 3.1 which has been working fine but in the last week or so I am unable to send emails. Emails are still being received but when I go to send one the error message Cannot send mail- a copy has been placed in your outbox. The recipient xxxxx was rejected by the server appears.
The only thing thats changed is I had changed the email I was using, but it did work for a couple of days before it just stopped.
How can I fix this? Any ideas?
Thanks in advance.
iPhone OS 3.1