You have a he-said-she-said case. The facts that you have described give you recourse for the landlor's breaching the rental agreement. You should be entitled to damages and/or specific performance, i.e. forcing the landlord to perform his obligations. If you seek damages, you may include the increased cost, if any, of renting a substitute vacation rental or other monetary loss or expenses that were the consequence of the landlord's breach.
On the other hand, if the landlord testifies that the check was past due and a court believes his story, then you would be left with no recourse. Your email, indicating that "the check is in the mail," may help, but it is not conclusive.
Be careful on this one. If you sue the landlord for damages and/or specific performance, the prevailing party to the action may be able to recover his or her attorneys fees, if there is an attorneys fees clause in the agreement. Most of the rental agreements I have seen include an attorneys fees clause.
You need to consider whether it is worth bringing an action, or are you better off just getting your money back (or cancelling your check, if it is not cashed).
