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.
Metta, Retro.
If you have asked me of the origination of unease, then I shall explain it to you in accordance with my understanding: Whatever various forms of unease there are in the world, They originate founded in encumbering accumulation. (Pārāyanavagga)
'We should not congratulate someone on the success of their misdeeds, but on the contrary should endeavour to advise him or her to lead a more skilful and wholesome life. If such advice is ignored then we can only give up and let go' - Phra Panyapatipo