"the message xxx could not be moved to the mailbox xxx" keeps popping up

This error message indicates that there is a problem with the email program on your Mac. It is possible that the email program is not functioning properly or that there is a problem with the configuration of your email account.

To troubleshoot this issue, try the following steps:

1. Restart your Mac and try composing the email again. This can help to resolve any temporary issues that may be causing the error message to appear.
2. Check to make sure that your email account is set up correctly in the Mail app. Open the Mail app and go to the "Accounts" section in the preferences. Make sure that your email account is listed and that all of the settings are correct.
3. If you are still experiencing the issue, try deleting your email account from the Mail app and adding it again. This can help to reset the connection between the app and your email server, which may resolve the issue.
4. If the problem persists, you may need to reinstall the Mail app on your Mac. You can do this by going to the "Applications" folder in Finder, finding the Mail app, and dragging it to the trash. Then, go to the App Store and download the Mail app again.

ive this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.