Q: cannot send mail, username or password is incorrect

I had this problem shortly after my email service provider (Cincinnati Bell Fuse) required me to change passwords.

I updated the password at the service provider and then, dutifully, updated the credentials in my iPhone (or so I thought).

(settings / mail,contacts,calendars / accounts / [my account at fuse] / incoming mail server / password).

After this, I was able to receive but not send/forward/reply to  email.  When trying to send I would get error message " Cannot send mail ...the username or password for the xxx.smtp server is incorrect"

Verified it was correct.  Spoke with the service provider and Apple...each pointing to the other as a source of the problem.  Uninstalled and reinstalled the mail account at least a couple times (with Apple's help) and problem was not resolved.  ........

Finally, today spoke with service provider one more time and found that there is a separate password for SMTP (outgoing) server (independent of the incoming mail server).  This password is buried a couple more levels down.  Once this is corrected, BOOM!  Everything works.   In my previous experience, I was unfamiliar with separate incoming and outgoing mail server passwords being required. 

(settings / mail,contacts,calendars / accounts / [my account at fuse] / outgoing mail server /  > /primary server / click on server name / outgoing mail server / password).