What's the different between "Outbox" and "Sent Message?"

[DeepNDaUndaworld]

Same as the title. I don't know what's the different between them as I've never met similar problem. Few information were found. Anyone helps? Does that means message failed to send?

[mrsun]

Everything is simple and logically clear

"Outbox" - messages sent by you, but not read by the recipient

"Send Message" - messages read by the recipient and moved to the archive

[DeepNDaUndaworld]

"Outbox" - messages sent by you, but not read by the recipient

"Send Message" - messages read by the recipient and moved to the archive

Okey.... I think I might just made it too complex