The letter מ, the opened mem, symbolizes מיתה. This is how it is in galus, where the opened mem alludes to the fact that death is open to all. This is in contrast to the time of בלע המות לנצח, He will eliminate death forever, when the mem will be closed and sealed up as we see where the prophet foretells of the elevated states of the world in the future: לם רבה המשרה ולשלום אין קץ על כסא דוד, towards abundant greatness and boundless peace [that will rest] on the throne of Dovid and his kingdom, to establish and sustain it through justice and righteousness, from now to eternity.
The anomalous final mem in רבהםל is truly unique as it is placed in the middle of a word!
Dovid Hamelech is mentioned in that pasuk that contains the word לםרבה. Now, the name דוד is comprised of two dalets and a vuv. The letter vuv serves as a connector as it means “and.” When we connect the two dalets of Dovid we get to the letter ם since if we look closely, we notice the letter ם is essentially two ד combined together. This is obviously no coincidence as Dovid is דוד וקיםמלך ישראל חי, Dovid is alive and enduring.
Now, Dovid serves as the rectification for Adam who brought death to the world. Indeed, אדם serves as an acronym for אדם, דוד, משיח.
