I would first look to your lease document - review the specific requirments of the due date (as you said the 10th) and whether any grace period is described. Also look for any language saying that the payment must be "postmarked" by the 15th versus "received" by the 15th. Second, if nothing is explicitly stated in the lease, then generally the mailbox rule or postal rule would be a defense against these late fees, so long as the payment did eventually arrive. Finally, it seems strange that you would be slapped with a year's worth of late fees with no prior notice or opportunity to pay them. In other words, you should have been notified after the first late fee was assessed - but they waiting 12 months? It seems very strange, and it does appear a strong case could be made to get you our of those late fees. Please feel free to contact me if you would like to discuss further or if you need any assistance.
This answer is for general advisory purposes only and does not establish an attorney-client relationship.
Look to your lease. In terms of late fees and the due date of your rent, in most leases the controlling date is not when you mailed the rent check but when it was received by the landlord. The mailbox rule generally applies to contract law cases and is used to establish the date on whicha contract is accepted.
DISCLAIMER: Brandy A. Peeples is licensed to practice law in the State of Maryland. This answer is being provided for informational purposes only and the laws of your jurisdiction may differ. This answer based on general legal principles and is not intended for the purpose of providing specific legal advice or opinions. Under no circumstances does this answer constitute the establishment of an attorney-client relationship. For legal advice relating to your specific situation, I strongly urge you to consult with an attorney in your area. NO COMMUNICATIONS WITH ME ARE TO BE CONSTRUED AS ARISING FROM AN ATTORNEY-CLIENT RELATIONSHIP AND NO ATTORNEY-CLIENT RELATIONSHIP WILL BE ESTABLISHED WITH ME UNLESS I HAVE EXPRESSLY AGREED TO UNDERTAKE YOUR REPRESENTATION, WHICH INCLUDES THE EXECUTION OF A WRITTEN AGREEMENT OF RETAINER.