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.
What is the final conviction that comes when radical attention is razor-edge sharp? That the object of the mind is mind-made (manomaya). (Ven. Ñāṇananda)
Having understood name-and-form, which is a product of prolificity,
And which is the root of all malady within and without,
He is released from bondage to the root of all maladies,
That Such-like-one is truly known as 'the one who has understood'. (Snp 3.6)