Outlook sometimes prompts for pass if multiple users use same mailbox

==== Required information ====
- iRedMail version: 0.8.6
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): LDAP-AD
- Linux/BSD distribution name and version: RH 6.5
- Related log if you're reporting an issue:
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I sometimes encounter strange error.
We have couple of mailboxes that many clients connects to at the same time.
Sometimes all of them get password prompt. In log I can see that their login attempt had failed.
Is it some limit ?
Or do you have any idea what can I check
Regards
Michal Franczak