Pendulum
The answer to the riddle is "Mailbox." It starts with the letter 'M,' ends with 'X,' and can contain an endless number of letters (as in mail) inside it.
mosaicist
madtv
carilium
marjoram
molasses
mininova
marjoram
'Moist'.
mourners mistakes
There is a minor planet called Mannheim, which starts with M, ends with M, and has eight letters.
momentum starts with the letter "M" and has 8 letters, while force starts with the letter "F" and has 5 letters.