Usage patterns
宛先を書くto write the address (on an envelope, package, etc.)
Please write the address on the front of the envelope.
宛先に送るto send (something) to an address
I accidentally sent this email to the wrong address.
宛先を確認するto check/verify the address
Please double-check the address before sending.
宛先不明address unknown (mail returned as undeliverable)
The mail came back marked 'address unknown'.
宛先を追加するto add an address (e.g. to a mailing list or address book)
I added a new address to my address book.