Migration error: "The server cannot service the request right now" (4241619)

If Microsoft EWS throttling has not been relaxed you may see the following warning when migrating mailbox data.

The server cannot service the request right now.  Try again later.  Waiting 5.00 minutes before resuming.

"[1/3]Error detected, attempting retry 1 of 5
An existing connection was forcibly closed by the remote host
(An existing connection was forcibly closed by the remote host)"
"[2/3]Error detected, attempting retry 1 of 5
An existing connection was forcibly closed by the remote host
(Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.)"
"[3/3]Error detected, attempting retry 1 of 5
An existing connection was forcibly closed by the remote host
(The request failed. Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.)"

Exception of type 'Quest.Rest.RestException' was thrown.; WebStatus = Success; StatusCode = ServiceUnavailable; StatusDescription = 'Service Temporarily Unavailable'; Content = ''

or

"Waiting for Microsoft Azure to send the response back, The server cannot service this request right now. Try again later. waiting 0.00 minutes before resuming"

"Response status code does not indicate success: 403 (Site Disabled).
(Response status code does not indicate success: 403 (Site Disabled).)"

Error creating source session. Response status code does not indicate success: 403 (Site Disabled).(Response status code does not indicate success: 403 (Site Disabled).)