Unable to open this mailbox

​Error: Unable to open this mailbox.

​This error indicates that the Source returned a folder which could not be opened. The term "mailbox" is confusing: in IMAP technical terminology, a mailbox is actually a folder.


There are different reasons why you may receive this error:

  • The user deleted, renamed, or moved the folder while the migration was in progress.
  • The folder has special characters or spaces, causing the IMAP system to fail when trying to open the folder.
  • The folder is a special system folder (i.e., anti spam folder) which can never be opened.

We recommend the following:

  1. Check to see if the folder still exists with the same folder path.
  2. Check to see if the folder contains special characters, or looks like a "special" system folder.
  3. Consider filtering the folder.
  4. Resubmit for migration.
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