invalid mail format

Emails are stuck in the POP3 Spooler Queue with the error message: invalid mail format.
These mails cannot be processed by the MailDepot Connector.

A possible reason for this is an ongoing Exchange Server update.
These mails are status mails with following content:
The attached message was sent by a member of this shared mailbox.
Usually, it would appear in Sent Items, but your service is currently being upgraded.
Things will be back to normal after the upgrade.
This means, that the mails are no journal mails at all, having neither a sender address nor a message ID.
The corresponding journal mails are already archived.

Wait upon completion of the exchange Migration and delete the mails with the "invalid mail format" error.