The antonyms for the word "marked with red letter" would be unremarked, unnoticed or unimportant. When a word or phrase is marked with red lettering, it usually means that it is significant, essential or deserving of special attention. Conversely, if something is unremarked or unnoticed, it means that it has been ignored, overlooked or is not considered noteworthy. Similarly, if something is unimportant, it is irrelevant, inconsequential or insignificant. Therefore, the antonyms for "marked with red letter" express the opposite sentiment and indicate that something is not noteworthy or doesn't require extra attention.