No, the Mitel's SMTP is fire and forget... it will try the SMTP gateway and looks for a 250 OK message, if it receives something 503 error, it will try again a few times, but once it gets a 250 OK, the Mitel is done. To my knowledge it cannot be configured to send multiple messages for a single email.
The only other possibility here is if your SMTP server is misbehaving, and accepting a message and queuing it for deliver but is responding to the Mitel with a code that is not 250 OK, so the 5000 things the message didn't go through and it tries again until it finally just gives up and discards the email, potentially sending several unacknowledged emails. This is where the mainmail.log file in the 5000 would be very beneficial, it would quickly show what is going on.