Unable to send email

We have just installed MDM and email is received. When a user tries to send an email (new, reply or forward) it processes for a while and then a message pops up:

'Cannot send mail, check the settings for the outgoing servers in Settings > Mail, Contacts, Calendars'

I have checked and don't see anything obvious. We are still on Exchange 2003.

Any ideas please?

Thanks

Andre