Are they domain joined computers? The users in particular that have this issue have they changed their password recently?
Feb 13, 2014 Let me refer to A as accessing my outlook.com email from within Mac Mail, and B as accessing my outlook.com email from the Outlook.com website from Safari. When I send an email to my outlook.com eamil address, I can see the email in both A. To add your email account to Outlook, you’ll need an app password. This is a different password than your regular email account password. The steps to obtain an app password are different for each email provider. The Outlook.com mail server settings are valid for every email client application that supports the POP3, IMAP or Active Sync protocols, no matter the platform (Windows, Mac, iOS, Android, etc). Outlook.com POP3 servers. With the addition of IMAP, I can finally set up Microsoft's free email service with Microsoft's mail app for the Mac. Launch Microsoft Outlook and select Tools > Accounts from the menu bar. If you don't already have an account set up in Outlook, this will appear automatically. Select E-Mail Account under the account type that you can add.
Outlook Email Server Settings For Att Imap
We have issues sometimes with this. Previously it boiled down to the wrong username being used. The events would go password expired whilst off the network, the Laptop wouldn't be updated with the new password, and then the prompts would appear and the user would just accept the username in the field which was their email address and not the UPN. They'd put in the password without changing the username, and it would fail.