You make 2 identical original letters, you sign both.
You hand both to your messenger (who can be anyone, e.g. a friend) and you then ask the messenger to read both, to ensure that they are both the same.
You then ask your messenger to throw one of them (the messenger chooses which) into the landlord's Briefkasten and to write by hand in blue on the other one, somewhere where there is space on this letter (not on a separate page!):
Hiermit bestätige ich, dass ich das Original dieses Schreibens am <insert date your messenger threw it into the Briefkasten> um <insert time your messenger threw it into the Briefkasten> Uhr in den Briefkasten von Hr. <insert name and address of landlord> eingeworfen habe.
<signature of messenger>
Unterschrift von <insert name of your messenger>