The following may be seen in Mailbox Shuttle:
Microsoft throttles ingestion into Office 365 mailboxes and Personal Archives. This can lead to slow migration rates with Mailbox Shuttle. In most circumstances, we are able to detect the messages that come back from Office 365 and the result of that is the information shows in the user interface.
The message indicates that throttling is taking place, and the time in which the actions from Mailbox Shuttle will be tried again. Seeing this message frequently may lead to a disruption of the migration, and that steps within the migration suffer delays.
The following article will help with the discussion between the customer and Microsoft, relating to throttling.