"A lease cannot be terminated by the unilateral act of the tenant." Brywood Ltd. Partners, L.P. v. H.T.G., Inc., 866 S.W.2d 903, 905 (Mo.App.1993). Under Missouri law, when a tenant defaults on a lease, the landlord has three options: "`(1) [r]emain out of possession, treat the lease as subsisting and collect rent; (2) give notice to tenant, resume possession of the premises and attempt to relet in order to mitigate any damages; or (3) reenter, resume possession in its own right and, effectively, terminate the lease.'" Blue Ridge Ctr. Ltd. Partnership v. Zadeh, 943 S.W.2d 357, 358 (Mo.App.1997) (quoting MRI Northwest Rentals Invs. I, Inc. v. Schnucks-Twenty-Five, Inc., 807 S.W.2d 531, 534 (Mo.App.1991)). Although, generally, a landlord has no duty to mitigate damages, once it accepts possession of the property and notifies the tenant of its intention to attempt to relet the property, the landlord voluntarily assumes a duty to mitigate, as the second option reflects. Id. 201*201 at 359. Once a lessor accepts the duty to mitigate damages, although it "is not required to do everything imaginable" to do so, its attempts must be reasonable. Brywood Ltd. Partners, L.P., 866 S.W.2d at 907. The tenant has the burden of proving that a lessor failed to take such reasonable steps to mitigate damages. Blue Ridge Ctr. Ltd. Partnership, 943 S.W.2d at 359.
...the RESTATEMENT (SECOND) OF PROPERTY § 12.1 cmt. i (1977) does provide that, if the landlord chooses to relet the property to mitigate the damages against a defaulting tenant, "the tenant is liable for the difference between the rental the tenant was obligated to pay and the amount received from the new tenant, and is also liable for the reasonable costs incurred by the landlord to procure a new tenant." To the extent necessary to prevent, in our view, the obvious injustice of a defaulting tenant from receiving the benefit of the landlord's reletting of the property without requiring him or her to be responsible for any expense incurred by the landlord in actual mitigation of his or her damages, we would adopt the Restatement position to the extent discussed, infra.