Keep getting send error when trying to send or reply to email

I have a Nokia Lumia 521 windows phone running windows 8.1 on which I have a few separate email accounts to manage. I am repeatedly receiving send error messages when using a non-gmail account, (outlook.com, mail.com). I've tried re-starting the phone, cleaning the cache, deleting messages from folders, but the results, if any are short-lived and unsatisfactory. Any suggestions or a fix would be welcome. Software version is Lumia Cyan
Phillip in CO

Have you tried deleting the accounts and re-adding them?