next flight
Used when referring to the next available flight, e.g., at an airport or when discussing travel plans.
What time does the next flight depart?
next mail; next post
Refers to the next delivery of mail or the next post in a series, such as in a blog or social media context.
I will send the documents by the next mail.
Phrase composed of 次 (next) + の (possessive particle) + 便 (flight, mail, convenience). The meaning depends on context.