。
Please mail this letter tomorrow morning.
Literal
Tomorrow [possessive-の] morning this letter [object-を] send-out please.
出す has many meanings depending on context — here with 手紙 it specifically means 'to mail/send out' a letter. Literally 'put out' or 'take out,' the verb's semantic range covers everything from mailing letters, to paying money, to serving food, to submitting documents. Context is everything.