Symptoms

A user configures a new Microsoft Outlook profile by using an email address that resembles the following:

con.user@xyz.onmicrosoft.com
In this situation, the following error message is returned:

Cannot start Microsoft Outlook. Cannot open the outlook window. The set of folders cannot be opened. Unknown error.

The screenshot for the error message

Cause

Outlook uses the email address as the name for the data file, as in con.user@xyz.onmicrosoft.com.ost. However "con" is one of the MS-DOS device driver names. Windows doesn’t let you use an MS-DOS device driver name when you create a file. The following is a list of the MS-DOS device driver names:

Name

Function

CON

Keyboard and display

PRN

System list device, usually a parallel port

AUX

Auxiliary device, usually a serial port

CLOCK$

System real-time clock

NUL

Bit-bucket device

A:-Z:

Drive letters

COM1

First serial communications port

LPT1

First parallel printer port

LPT2

Second parallel printer port

LPT3

Third parallel printer port

COM2

Second serial communications port

COM3

Third serial communications port

COM4

Fourth serial communications port


Resolution

To resolve this issue, use one of the following methods.

Method 1: Change the email address of the affected mailbox on the server side 

Method 2: Change the name of the data file in the Outlook profile

To do this, follow these steps:

  1. Exit Outlook.

  2. In Control Panel, open the Mail item, and then click E-mail Accounts.

  3. Click Change, and then click More Settings.

  4. On the Advanced tab, click Outlook Data File Settings.

  5. Select the File name, and then delete the period (.) after the aux or con prefix. For example, if the file name is aux.user@fourthcoffee.onmicrosoft.com - aux.user.ost, change it to auxuser@fourthcoffee.onmicrosoft.com – aux.user.ost.

  6. Click OK.

  7. When you're prompted for confirmation, click Yes.

  8. Click OK.

  9. In the Change Account window, click Next, and then click Finish.

Outlook will now start correctly.
 

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