Colorado's law only says that a written notice must be mailed. Their testimony that this was handled in the normal course of there business should be enough to show that it was mailed. Joe Crandall's suggestion that you obtain a copy of the statement is valid, but you can ignore his advice that you can claim treble damages in this case. The LL has already told you that the statement was mailed and that you still owe money, and they will testify to that in court.
Did you provide the landlord your forwarding address when you moved? If not, then the LL could have met the legal requirements by simply sending it to your last known address (the unit you were renting.)
See
https://tinyurl.com/yc5aq46s for Colorado's security deposit law.
If it means that much to you, make the landlord prove he mailed it. The original letter in the original envelope still sealed inside, needs to be stamped with a date certain that would be in the proper time frame. And the address he sent it to must be an address that he reasonably can show as a probable address, that you did live at, regardless if past or present.
And yeah, there is dishonesty in Colorado Courts, probably in all courts. We cannot change that.
C.R.S. 13-21-102 (1)(a) In all civil actions in which damages are assessed by a jury for a wrong done to the person or to personal or real property, and the injury complained of is attended by circumstances of fraud, malice, or willful and wanton conduct, the jury, in addition to the actual damages sustained by such party, may award him reasonable exemplary damages. The amount of such reasonable exemplary damages shall not exceed an amount which is equal to the amount of the actual damages awarded to the injured party. . . (3) Notwithstanding the provisions of subsection (1) of this section, the court may increase any award of exemplary damages, to a sum not to exceed three times the amount of actual damages, if it is shown that: . . .(b) The defendant has acted in a willful and wanton manner during the pendency of the action in a manner which has further aggravated the damages of the plaintiff when the defendant knew or should have known such action would produce aggravation.