Microsoft Office 365 customers most frequently report having trouble connecting Outlook to Exchange online. In most of the documented cases, this particular problem affects a small subset of the user base. There is no definitive fix for the problem, but there are a number of things that can resolve an Outlook connectivity issue for some Office 365 customers.
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.
What do I do if the mailbox is corrupt?
If the mailbox appears corrupt, you'll have to repair the corrupt mailbox or delete and re-create it. Either way, data loss is inevitable. Determine beyond a reasonable doubt that the mailbox really is corrupt before you go through a process that will most likely result in data loss.
You'll also have to verify three things. First, make sure Outlook connectivity is working for other users in your organization. Second, make sure the user having problems doesn't already have a profile on the PC you're testing Outlook connectivity on; profile problems are a common cause of Outlook connectivity failures. Third, make sure the PC you're testing has the same operating system, the same Outlook version and the same patches as PCs known to be providing proper Outlook connectivity.
Some backup applications support the granular recovery of Exchange Server data. If you're forced to delete and recreate a mailbox, you may be able to use your backup application to populate the new mailbox with the user's data.
About the author
Brien Posey is an eight-time Microsoft MVP for his work with Windows Server, IIS, Exchange Server and file system storage technologies. Brien has served as CIO for a nationwide chain of hospitals and health care facilities, and was once responsible for IT operations at Fort Knox. He has also served as a network administrator for some of the nation's largest insurance companies.