Usage patterns
〈物〉に封をするto put a seal on something
I sealed the letter and sent it.
封がしてあるto be sealed (state)
封を解くto break or undo a seal
I carefully broke the seal.
封が破れるthe seal tears/breaks
The seal had torn during delivery.
Expressions
別封でunder separate cover; in a separate envelope 封を切るbreak the seal; open a sealed item