It's because the recipient hasn't access their messages in order to pick them up.
Once you see them drop out of the outbox, you will know they have been received.
Being in the outbox also means you can edit the message if you like, and also delete it if it's no longer of any relevance.
"Do not force others, including children, by any means whatsoever, to adopt your views, whether by authority, threat, money, propaganda, or even education." - Ven. Thich Nhat Hanh
"The uprooting of identity is seen by the noble ones as pleasurable; but this contradicts what the whole world sees." (Snp 3.12)
"To argue with a person who has renounced the use of reason is like administering medicine to the dead" - Thomas Paine