Five ways to fix Outlook connectivity issues

1. Confirm that Outlook is actually the problem.

First, you need to confirm the problem truly is related to Outlook. Identify which users are experiencing the problem, and then use an alternate method of connectivity to open those users' mailboxes. For example, you might try accessing a user's mailbox through Outlook Web App (OWA) or through Exchange ActiveSync.

If the mailbox is accessible through OWA or Exchange ActiveSync, it means the problem is most likely Outlook-related. If it isn't, the problem is more likely to be related to an Office 365 connectivity issue or related to a corrupt mailbox.

2. Try a different PC.

Once you've established Outlook as a probable cause for the connectivity problem, try using Outlook to open the user's mailbox on a different PC. If you can open the user's mailbox on a different computer using Outlook, you've verified the user's mailbox integrity. If you can't open the mailbox on any PC but Outlook works for other mailboxes, there may very well be a mailbox corruption issue at work. But there are still other things to check.

3. Reset the user's password.

Try resetting the user's Office 365 password. Even though it might be counterintuitive for a password reset to correct an Outlook connectivity problem, there have been a number of documented cases in which users could suddenly use Outlook to connect to their mailboxes once their Office 365 password was reset.

4. Delete the user's profile.

Try deleting the user's local profile on the PC they use to connect to Office 365. There are many reports of user profile corruption that result in Office 365 connectivity issues. Most of these reports center on the Outlook OST file.

If the OST file is to blame, deleting the user profile should correct the connectivity issue because the OST file is a part of the user's profile. There isn't a definitive explanation for why the OST file becomes corrupted, but some speculate it may be a result of the way Outlook 2013 compresses OST file contents.

5. Try the Microsoft Online Services Sign-In Assistant.

If you've been unable to resolve the Outlook connectivity problem so far, install the Microsoft Online Services Sign-In Assistant. This tool technically isn't required, but some find installing it corrects connectivity issues for Exchange Online. It's worth noting that the Microsoft Online Services Sign-In Assistant is not compatible with Windows 8. You can download the tool here.