You’re repeatedly prompted for Exchange credentials after you sign in to Lync

If a firewall or a proxy is blocking Lync from connecting to EWS, you may experience symptoms such as repeated credential requests, stale Address Book Service (ABS), and intermittent Free/Busy presence issues. To resolve this problem, verify that the user has the correct proxy configured in Internet Explorer. To do this, follow these steps:

1. Start Internet Explorer.
2.  On the Tools menu, click Internet Options, click Connections, and then click LAN Settings.
3. Make sure that the Automatically detect settings option is selected. If your organization requires you to enter specific information for the proxy server or an automatic configuration script, contact your network administrator.
4. Restart both Internet Explorer and Lync to check whether the problem is resolved. If the problem persists, go on to the next section.

Lync can’t connect to Outlook or EWS

If Lync doesn't integrate automatically with Outlook, Lync prompts you for credentials before it lets you connect to Outlook to retrieve Calendar and Free/Busy information. If you are prompted repeatedly after you enter your credentials, see the following Microsoft Knowledge Base article: If Lync doesn't automatically connect to EWS, Lync prompts you for credentials before it lets you access the EWS URLs. If you are still prompted after you enter your credentials, see the following Microsoft Knowledge Base article:If you're repeatedly prompted to enter your credentials or receive the "Access Denied" error message, see the following Microsoft Knowledge Base article:

Lync can’t connect to Outlook or EWS

If the Exchange server or EWS is unavailable, this causes Lync to continuously prompt for credentials because it can't contact Exchange for authentication. To resolve this issue, verify that Outlook has connectivity to the mailbox. If Outlook can't connect to Exchange, troubleshoot the issue as an Exchange connectivity issue. For more information, see the following Microsoft Knowledge Base article:If a firewall or a proxy is preventing Lync from connecting to EWS, you may experience symptoms such as repeated credential requests, stale ABS, and intermittent Free/Busy presence issues.

Ports 443, 5060, and 5061 must be open on the firewall and on proxy servers to let traffic pass freely. For more information about firewall or proxy issues in an Office 365 environment, see the following Microsoft Knowledge Base article: