Harlem
Harlem
Those six letters are "amount."
mimosa
marshes
amoeba
reqium reqium
erbium
bottom
Harlem
malice, mousse
· Malawi · Mexico · Monaco
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.