[exim] Cant authenticate using any email client

Hi All

iam running exim on my outbound server which authenticates using
courier-authlib

For some users, i can authenticate perfectly on webmail but cant on any
email client

Error i receive when i try is "535 Incorrect authentication data"

This only happens when using any email client say microsoft outlook.

What could be the issue??

Awaiting your quick response

Regards
Ritah