We don't have synonyms for day letters yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are day letter, dead letters and fan letters.
(n) :
(telegraphy, historical) A telegram sent during the day for slightly deferred delivery on the same day, cheaper than a fast telegram, typically with a minimum charge for 50 words or less.
(n) :
(telegraphy, historical) A telegram sent during the day for slightly deferred delivery on the same day, cheaper than a fast telegram, typically with a minimum charge for 50 words or less.