If you have received the error Rejected by SLM Monitor when creating a Canada Post label, this is due to Canada Post's servers throttling your Create Label request.
Typically, Canada Post allows 20 requests per minute. If you receive this error frequently, you can request a limit increase on your Canada Post account.
You may also receive this error in the event of a Canada Post server maintenance/outage. In either instance, if you receive this error, your best option is to give it a couple of minutes and try again.