New Year's mail
Mail sent during the New Year period, typically including nengajō (New Year's greeting cards).
New Year's mail is delivered on New Year's Day.
年賀状 specifically refers to New Year's greeting cards, while 年賀郵便 is the broader category of New Year's mail.
Compound of 年賀 (New Year's greetings) and 郵便 (mail, postal service).