Fetching details from the dictionary...
opening (a letter, parcel, etc.); breaking the seal
unboxing (a product)
unsealed mail
手紙を開封して中身を見た。
I opened the letter and looked at its contents.
開封後はすぐに返事を書いた。
After opening it, I immediately wrote a reply.
開封せずに返送してください。
Please return it without opening.