Symptoms
When copying mailboxes with external calendar events, ShareGate Migrate displays the following error:
MC-063-503: Something went wrong. The external user's forwarded event could not be retrieved.
Error code
MC-063-503
Details
When ShareGate Migrate encounters an external event in a mailbox, Mailbox copy forwards the event to the destination mailbox instead of copying it. It then retrieves this event from the destination mailbox and sets it up.
If the source or destination mailboxes have any policies in place that are preventing this forwarded event from being delivered, Migrate cannot retrieve it, and the event will not be migrated. Any DMARC/SPF records they have in place can also cause this behavior.