ALS
Autograph Letter Signed

anonymous
anonymous letter

condolence
letter of condolence

covering
covering letter

credence
letter of credence

DLO
dead letter office

enclosed
enclosed letter

four-letter
four-letter word

gratulatory
a gratulatory letter

hinge on
...on the letter he is going to write

lc
letter of credit

letter
capital letter

letter-bound
preso à letra

letter-perfect
the actor was letter-perfect in his role

Letter-winged Kite
Elanus scriptus

marque
letter of marque

missive
letter missive

monitory
monitory letter

perusal
the perusal of a letter

poison-pen
poison-pen letter

recommendation
letter of recommendation

recommendatory
recommendatory letter

recorded
recorded delivery letter

red-letter
red-letter day

registered
registered letter

stopped
a badly stopped letter

ultimo
your letter of the 15th ult.

unanswered
her letter remained unanswered

unmentioned
he left my letter unmentioned

unregistered
unregistered letter