カイ, とど.ける, とど.け, とど.く
deliver, reach, arrive, report, notify, forward
JLPT2 G6 S8 F939
凷 is 土 an abbreviated form of 圭 (clay/earth piled high in a neat cone shape) + 凵 hole → lump of earth (compare 圣 as described in 怪) that fills a hole. 届 is 凷# + 尸 body → person arriving at the top of a hill → *arrive*; *reach* → *send*; *pass along* *deliver*. Also, *report to*, *notification*, *registration*, *attend (to)*, and *attain*.
[Full Kanji Etymology List]