Password required, Server says "Incorrect authentication data"

Pop-up window appears saying Password required.  Have read numerous comments from myriad people with same issue, but can’t seem to find a solution.

The password is correct.  Have entered and re-enterd it many times.  The issue has been intermittent, but now id chronic and I can’t even complete an outgoing message without the window popping up. 

Our server management team can find no problem and we’ve change IMAP and
SMPT port as a test.

Does ANYONE know of a solution?

thnx, Mike