NOT A TEACHER
(1) As one moderator said, the other moderator's sentence is fine.
(2) I know that you are a very serious student, so I congratulate you on wanting
to learn "perfect" English.
(3) If you look carefully at the moderator's sentence, we see two sentences:
(a) Take a look at an earlier letter that I sent you.
(i) The moderator did not use the relative pronoun "that" (whose antecedent is "letter").
Native speakers often do not use the relative pronoun in such sentences.
(b) Take a look at an earlier letter that was dated May 14.
(i) This time we cannot forget the relative pronoun, for it is the subject. As you know,
it would be bad English to say: Take a look at an earlier letter was dated May 14.
(The first "that" can be "forgotten" because it is "only" the object of "sent.")
(ii) But we could say: Take a look at an earlier letter dated May 14.
(4) Thus we have a wonderful choice:
(a) Look at an earlier letter that I sent you that was dated May 14.
(b) Look at an earlier letter I sent you that was dated May 14.
(c) Look at an earlier letter that I sent you dated May 14,
(d) Look at an earlier letter I sent you dated May 14,
(5) I am sure that your first language also allows speakers to use a variety of ways to express the same idea. Am I right?