Question: What is the "mailbox rule?"
Answer: A payment under a contract (such as rent) is deemed delivered when it is received. However, according to Colorado case law, if the parties to the agreement have established a course of dealing that payments are delivered and accepted by mail, then a payment sent by mail is deemed delivered when it is placed in the U.S. Mail with proper postage and properly addressed.
So when, after a failure to pay rent by a resident, the landlord serves a three day Demand for Compliance or Possession, in order to timely comply, the resident must pay the rent in full by the end of the third full day after the date of service of the Demand. However, if rent is usually paid and accepted through the mail, then so long as the resident gets the rent check into the U. S. Mail (i.e.: post-marked) by the end of the third day, it can be deemed by the court to have been timely paid even if the landlord doesn’t actually receive it until after the third day.