As a MigrationWiz user, you decide how many errors a migration can encounter before it is marked as failed. Simply specify the error limit in advanced connector settings (the default value is 100 errors). This means that a migration can succeed overall, but also report that some errors were encountered.
For example, suppose that while migrating a large mailbox the destination server was down for an hour. While your migration may have succeeded overall, you’ll probably see something like this in migration error logs:
What this tells you is that some items failed to migrate due to connectivity issues. However, MigrationWiz makes it easy to retry failed items. This option is always free, but can be used only for migrations which completed successfully with errors.
Handling migration errors:
In our experience, most mailbox migrations complete without any error. However, consider a mailbox with 100K items. Even with a low failure rate of 0.01%, you’re looking at approximatively 10 errors. It is therefore reasonable to expect some errors when migrating large mailboxes. The key question is: how does MigrationWiz handle errors?
Error handling is an essential part of what we do. To handle errors gracefully, we employ different techniques such as exponential backoff retries, repairing corrupt email content, quarantining failed items, classifying errors as retriable vs. non-retriable, etc. In most cases, each mailbox item is given several chances to migrate. However, after all attempts have been exhausted, the migration process moves past the failed item, until the migration completes or too many errors are encountered.
Common migration errors:
The most common types of migration errors are caused by server outages and invalid credentials. Excluding those obvious ones, here are some interesting ones:
– “Some messages could not be FETCHed”. This error occurs when migrating corrupt emails from Gmail. We believe this error mostly affects emails received in Gmail before it started supporting IMAP, causing its storage engine to lose header information. Also, keep in mind that Gmail uses commodity computing and provides no guarantees in terms of data integrity or protection against data loss,
– “Failed to create items under folder X: ErrorMimeContentInvalid”. This error occurs when trying to migrate corrupt e-mail content to Exchange. Exchange Server examines all inbound emails and rejects those whose content does not adhere to specifications. In some cases, we are able to automatically detect and fix corrupt MIME content, but not always.
– “The remote server returned an error: (507) Insufficient Storage”. This error occurs when a mailbox item is larger than what Exchange configuration parameters allow. In most cases, this error can be fixed simply by increasing Exchange limits and retrying migration errors. However, if your Exchange server is hosted by a third-party provider, such limits may be outside your control.