Knowledge Base

Invalid mime content


During the migration with CodeTwo Exchange Migration or CodeTwo Office 365 Migration some items are not migrated and one of the following errors is recorded for them in the log files:

Error while uploading package. System.Exception: [ErrorMimeContentInvalidBase64String] - Invalid base64 string for MIME content.
Error while uploading package. System.Exception: [ErrorMimeContentInvalid] - Invalid MIME content. 


Both of the mentioned errors are caused by the invalid MIME content of the message - this basically means that items are corrupted. Such items may be fully accessible in Outlook or OWA but are considered impossible to be migrated as some of their properties invisible to the end-user, but required for migration, are missing or broken. For your information, it is the requirement of the target EWS service of MS Exchange Server (either On-premises or Online) that migrated items are in good health. CodeTwo Migration software attempts to repair corrupted items, although these attempts are not always successful and then, target server's EWS rejects such corrupted items.

From our experience, MS Outlook has the ability to repair corrupted items. You might want to try using MS Outlook to fix such items if the problem is limited to just a few of them, again in just a few mailbox. To do so, please follow the steps below:

  1. Create a folder on your computer (e.g. on a desktop) in which you will temporary store items with the corrupted MIME content.
  2. Open MS Outlook.
  3. Find the corrupted messages in your mailbox. They will most likely work like a charm in MS Outlook and do not really show any signs of corruption. Review the CodeTwo Migration software "mailbox" log files to pinpoint the corrupted items first.
  4. Drag and drop corrupted items to the temporary folder (see point 1) from the MS Outlook.
  5. Permanently delete the corrupted items from MS Outlook. Make sure you do not move those items to the Trash/Bin by simply hitting Delete button, use Shift+Delete upon selecting them to remove them from the mailbox completely.
  6. Now, drag and drop exported items back to the MS Outlook, from the temporary folder (see point 4). Pay attention when doing so, to copy items into the same folder of your mailbox that the original items were taken from.
  7. Give it a minute or so, to synchronize Outlook with the Exchange Server.
  8. Close MS Outlook and use the Run delta migration button in CodeTwo Migration software to restart the migration.

If the above steps do not help or there are too many corrupted items for this workaround, it might be necessary to exclude those items from the migration.

CodeTwo will appreciate if you provide examples of such corrupted items in a PST archive for analysis by our developers. We would like the software to be able to auto repair such items but it is not possible to cover all possible corruption scenarios without appropriate examples. Use our file upload form to provide zipped PST file with examples along with the software diagnostic details. Hopefully, we will be able to implement a software-side fix for your corrupted items in the next release of the software.

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