Unsettled wrote:

I should clarify - I'm referring to the case where people recommend to
create a new PST and change around the default delivery location.

What is the technical reason for this? Sure, I know it works, but what
is going on? Does it have to do with locking of the file?

There are a number of things that can cause messages to be undeliverable.
Outlook will continue to try to deliver them. If the reason they're
undeliverable (for example, the account they're trying to use is no longer
available because it has been deleted, an antivirus scanner has disrupted
the communication path between Outlook and the SMTP server, or the ISP has
made a change and didn't tell the client, to name a few reasons) cannot be
rectified, but Outlook has the message locked in its continuing attempts to
deliver it, then you have to change the situation. The simplest way is to
set Outlook to work offline, which after stopping and restarting, will have
the message in the Outbox deletable because Outlook will not be trying to
send it, Outlook being offline. The worst case will be that you have to
create a new delivery location. That new locations Outbox won't have
anything in it and, since OUtlook will send only from the delivery
location's Outbox, the old Outbox will be just another folder and whatever
it contains will go away when the PST itself goes away.
--
Brian Tillman