Salutation is the term used to describe the beginning of a letter or other correspondence. What is the term used for the closing of a letter? Here are some examples: Yours truly, Sincerely, Best wishes, Love?
Searching for 'antonym of salutation'1 was not helpful.
EDIT: This need not be expressed as a single word. (Whatever is correct is fine, whether one word or a phrase!)
1. Citation (IEEE Style): Thesaurus.com, "salutation," in Roget's 21st Century Thesaurus, Third Edition. Source location: Philip Lief Group 2009.