Your mailbox is temporarily unavailable because another e-mail message is being delivered to it (924244)

Source: Microsoft Support

RAPID PUBLISHING

RAPID PUBLISHING ARTICLES PROVIDE INFORMATION IN RESPONSE TO EMERGING OR UNIQUE TOPICS, AND MAY BE UPDATED AS NEW INFORMATION BECOMES AVAILABLE.

SYMPTOMS

When trying to log into a mailbox either through Mapi or POP you receive the following error: Your mailbox is temporarily unavailable because another e-mail message is being delivered to it or another mail application is accessing it. The server responded: -ERR The referenced account is currently locked out and may not be logged on to. In looking in the App log you can see the following event being logged;

Event Type: Error 
Event Source: POP3SVC 
Event Category: Authentication 
Event ID: 1011

Error code 0x775

RESOLUTION

The Error code 0x775 translates to (Windows - account lockout). check the account to make sure it is not locked out and in this particular case there was a duplicate AD account that was locked out.

DISCLAIMER

MICROSOFT AND/OR ITS RESPECTIVE SUPPLIERS MAKE NO REPRESENTATIONS ABOUT THE SUITABILITY OF THE INFORMATION CONTAINED IN THE DOCUMENTS AND RELATED GRAPHICS PUBLISHED ON THIS WEBSITE FOR ANY PURPOSE. THE DOCUMENTS AND RELATED GRAPHICS PUBLISHED ON THIS WEBSITE COULD INCLUDE TECHNICAL INACCURACIES OR TYPOGRAPHICAL ERRORS. CHANGES ARE PERIODICALLY ADDED TO THE INFORMATION HEREIN. MICROSOFT AND/OR ITS RESPECTIVE SUPPLIERS MAY MAKE IMPROVEMENTS AND/OR CHANGES IN THE PRODUCT(S) AND/OR THE PROGRAM(S) DESCRIBED HEREIN AT ANY TIME.

For more information on the terms of use, click on the link below:
http://support.microsoft.com/tou/