noun
first flight; maiden voyage
Refers to the inaugural flight of an airline route or the first voyage of a ship. Often used in travel contexts.
新路線の初便は満席だった。
The first flight on the new route was fully booked.
noun
first letter; first correspondence
Used for the first letter or message in an exchange, such as a pen-pal relationship or business correspondence. Less common than sense 1.
文通の初便を書くのは少し緊張する。
Writing the first letter in a correspondence makes me a little nervous.