The Hebrew letter mem, both the “open” mem and the “closed” mem (at the end of the word) relate to exile and redemption.

Rabbeinu Bachye expands on this theme1 :

In Tanach it states2 : “L’MARBEH — (this is a closed mem) — “For promoting the increase of the government, and for peace without end, upon the throne of David and upon his kingdom.” Here, the mem (in L’marbeh) (in the middle of a word), which should be open, is closed to indicate that the full glory of Jewish government is concealed during the era of exile. In another verse,3 we find an open mem at the end of a word,

“I was viewing the walls of Jerusalem” — Nechemyah states — “which haim (they; this mem is open in the text) were broken down, and the gates thereof were consumed by fire.” The Midrash comments:

“When the walls of Jerusalem will be closed (and solidified), unlike now when they are open and cracked during exile, then shall the Jewish government be fully opened (with full unrestricted power) unlike now when it is closed (and restricted).

Thus both mems, written in an unusual manner in these two verses, allude to the Messianic era.

On the verse in Isaiahl’marbeh — with an enclosed mem (“For promoting the increase of government”) the Talmud comments:4

G‑d wanted to close up and bring an end to all Jewish adversities by designating King Chizkiyah as Moshiach. Yet, the attribute of justice claimed: ‘Here is Chizkiyah, for whom You performed so many miracles and yet, he failed to offer a song unto You.’ With this, his fate as Moshiach became closed and concealed.

In the Oral Torah, too, we see the significance of mem:

The six Orders of Mishnah begin and end with mem. There is an open mem in the beginning of Mishnah — Me’eymasai,5 and a closed mem at the end of Mishnah, in the word shalom, as it cites the verse,6 “G‑d shall bless His people with peace.” Thus these two mems will usher in a period of everlasting peace.

Moshiach’s name, too, begins with a mem (similar to the beginning and end of the Mishnah). Thus, our Sages tell us:7 “The exiles will be ingathered in the merit of (studying) Mishnah.”