How to stop that? Yosie 10.5.5

I assume you mean 10.10.5.

Under Mail > Mailbox you should see an option to "Rebuild". Clicking on it maaay correct your problem.

Edit: Forgot to say to select a mailbox in your sidebar before hitting "Rebuild".

Mail âž­ Preferences âž­ Accounts âž­ Mailbox Behaviors âž­ Sent Mailbox and select the mailbox you want to use for sent mail from the drop down menu.

El Cap/Mail 9.3 (3124), and I don't see a drop-down that offers a selection of mailboxes at the end of your path.

It shows up in Mail 10 in MacOS 12. I have become so accustomed to it that I didn't think to check it out in OS X.

When all else fails, I can often find a hidden or unpublished/unpublicized preference setting in MacPilot that will do the trick so I looked in MacPilot on a system running El Capitan and there was no preference listed to change the mailbox used to store sent messages — or to change an other target mailbox for that matter. 👹Never the less, it will show up in Mail 10 when you upgrade to Sierra in a few weeks.

In the meantime, if Artie's suggestion of rebuilding the mailboxes does not work — remember each mailbox must be rebuilt individually — and If this is an .iCloud, .me, .mac or any other Apple account...

1. Log onto the iCloud web interface
2. Open Mail
3. Click on the âš™ (gear) icon at the bottom left of the Mail window and select Preferences from the dropdown menu
4. Click on "General" at the top of the Mail Preferences window
5. In the row labeled "Mailboxes" ✔ the checkbox labeled "Save sent messages in" and select the desired mailbox from the list of mailboxes in the drop down menu to the right of that.
6. click on Done at the bottom right of the iCloud Mail Preferences window
7. Back on the Mac on the Mail menu bar select Mailboxes âž­ Synchronize All Accounts

This should work because it is independent of the operating system version on the Mac — and yes I just verified that the setting is there and works whether you are running El Capitan or Sierra. (I try not to make the same mistake twice, I make enough on the first time around.)

There may be similar preference settings for other email providers but there are too many for me to attempt to go into here.

Yet another solution to this problem — Airmail 3. Airmail 3 is a highly configurable email client and offers the opportunity to specify what Mailbox is mapped as the Inbox, Starred, Important, Drafts, Spam, Trash, All Mail, Archive, Sent, To Do, Memo, Done, and Snooze. And if you have multiple accounts the settings are unique for each account.

"What is the Snooze mailbox?", I asked when I saw it. Snooze is a feature where messages can be tagged manually or even by a rule to "snooze" until Later Today, Tomorrow, This Evening, This Weekend, or Next Week and it will go to the Snooze mailbox only to pop back up at whatever time was specified for each of the Snoozes.