If it was sent to you via email, it was in writing. You did not indicate if you have a copy of that email. If you do, print it out. If not, it is discoverable.
It would be helpful to know how much is at stake here (and some more facts in general), but I think you have a strong basis for recovery based on contract, especially if you can show reliance on the agreement. Also, you did not discuss the circumstances of your departure, but under a quasi contract theory of unjust enrichment you probably have a cause of action in restitution. They can't work you up to closing and then fire you before the commission is to be paid.
I would suggest you talk with an attorney locally who is well versed in labor law (I'm not) as there may be additional protections afforded you by the labor code.
It sounds like you might be entitled to your commission both because it has vested right about the time of your separation and considering the length of time it took to close the project, you should receive the bulk or all of your commissions.
If you have a written commitment from the company to that effect, your case is even stronger.
Thanks, and feel free to follow up.
Assuming that you do not have a written commission agreement that requires you to be employed to be eligible for commission pay, it sounds like you may have a claim against your former employer for the commissions on the deal that closed after your termination. You may want to file a claim with the Division of Labor Standards Enforcement.