Many times if you have problems sending emails, it is an issue with the settings in the outgoing mail server. Having the same problem on all 3 devices seems to be pretty odd, unless you synced the incorrect settings from your computer using iTunes.
However, you should check the setting on your devices. Check the outgoing mail server setting. Make sure that your username and password are in there.
Settings>Mail, Contacts, Calendars>Your email account>Account>Outgoing mail server - tap the server name next to SMTP and check in the primary server and make sure your username and password are entered and correct - even if it says that the password is optional.
If that isn't the problem, post back with who your email provider is and any more specific details that you can provide.