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
"Whether I were to preach in brief, Sāriputta, or whether I were to preach in detail, Sāriputta, or whether I were to preach both in brief or in detail, Sāriputta, rare are those who understand." (A I 333, Sāriputtasutta)