See the discussion that has been ongoing for 8 years here:
https://www.phpbb.com/community/viewtopic.php?f=64&t=675335&p=14497471#p14497471

Also see how many google hits for "phpbb outbox stuck" there are.

The default names of "outbox" and "sent" lead to much confusion for new users of phpbb.

In the physical outbox world (paper in boxes on a desk), something that is still in your outbox hasn't been sent to the recipient yet.

In the email outbox, something that is still in your outbox hasn't been sent over the network to your recipient yet.

In the phpbb outbox, something that is still in your outbox doesn't have any issues with delivery or sending. It's just a status indicator essentially showing that the recipient(s) haven't opened it yet. If I understand correctly you can still edit it if your board is configured to allow this. The recipient may not have seen the contents yet either (unless the board emails out notifications with the PM contents).

As evidenced from the thread above and google searches, "outbox" is a very confusing term for NEW phpbb users.

It requires explanation that "phpbb outbox just means the message hasn't been opened by the other user yet, you can still edit it (maybe) and don't worry, it's actually "sent" you don't need to send it again."

This happens over and over again all over the internet. For every thousand people posting about this there are probably many fold more who silently try to "fix" it themselves by deleting/resending PMs, etc.

Once you are no longer a NEW users, there's no issue if someone has explained it to you.

If the name was more intuitive it would require no explanation though.

I don't know what the best name would be, but something like:
Outbox --> Sent (unopened)
Sent messages --> Sent (opened)

or unread/read or "still editable" or "editable" or whatever ...

Anything intuitive and explanatory would avoid confusion for NEW users, which places a burden on OLD users who have to explain it over and over and over and over again because the use of the term "outbox" doesn't fit the "prior art" metaphors from the electronic mail and physical message realms people know of when first coming to phpbb.

Another suggestion on the linked phpbb forum post was that perhaps the banner text
for outbox/sent could describe by default what function they serve, so as to explain how
they are different from what a new users would expect an "outbox" is.

I would personally go with a new name rather than outbox, but perhaps the banner text
idea is a more "doable" change politically or what have you.

The goal here is just to provide as smooth an experience as possible to NEW users of phpBB PMs and remove any source of confusion they may have which results in lots of chatter on the boards to "resolve" the issue (which just gets resolved by an explanation of how it works, which could have been done in the first place by using a different label text or banner text )