Antonyms for the phrase "letter introduction" might include "closing remarks," "farewell," or "conclusion." These terms all indicate the end of a letter, rather than the beginning. Other possible antonyms could be "postscript," "addendum," or "afterthought," which indicate additional thoughts or information added after the letter has been written. "Absence" or "omission" could also be considered antonyms, as they suggest the lack of an introduction or an incomplete letter. Regardless of the antonym used, it is important to remember that the letter introduction is a critical component of any written correspondence, setting the tone for the rest of the communication.