[Motherly]

A breakdown of the messaging system through the board:

• Inbox: Messages sent by other members on the board that you have received.
  • Messages you have not opened are highlighted to draw your attention to them.
  • Messages you have replied to successfully have thin white lines that appears on both the left and the right in the basic Inbox screen list.
• Sent messages: Messages that were successfully sent by you through the board but have not been opened/read by the recipient yet.
  • These messages are automatically moved to the Read by recipient folder once they are opened by the recipient.
• Read by recipient: Messages that were successfully sent by you through the board and have been opened/read by the recipient.

In short, if your sent message doesn't appear in the "Sent messages" folder then check the "Read by recipient" folder. Sent messages just haven't been opened and read yet. Please keep in mind that there is no mandatory rule that your messages must be replied to by the recipient.

If the message does not appear under either folder then something happened and it was not successfully sent. You probably lost internet connection, the entire site server went down, or you received an error-message on the screen that said something else happened. (All of those possibilities are decently rare and shouldn't be expected.)
I have pretty good faith that if the system showed you a "sent" message then it was indeed sent and did not encounter any sort of error in the processing of such.