Of every setting involved in the "no mail after a PST change" problem, the default data file is the master switch. POP delivery follows it, and a PST change flips it silently. This page walks the Data Files tab slowly: how to tell your real mail file from a newcomer by size and date, how to move the default tick, and why the restart afterwards is not optional.
Names on this tab are unreliable - files get renamed, restored and copied until two of them sound identical. So this page teaches the identification habit as well as the clicks: the file that is large, modified today, and whose Inbox holds your newest real mail is your file. Set that one as default and delivery follows.
Example values. Size and modified date are the honest clues: your real mail file is the large one that changes daily, not the tiny file created the day things broke.
Do not guess from names alone - names lie and files get renamed. Judge each file by the evidence on this tab:
This step is skipped constantly and causes most "I changed it and nothing happened" stories. Delivery settings are read when Outlook starts. Close Outlook fully - File, Exit, not just the window if Outlook minimises to the tray - wait a few seconds, then open it again. Send yourself a test mail from your phone and watch it land in the correct Inbox before declaring victory.
The default data file is not only about incoming mail. For POP accounts it is the delivery home unless an account has its own delivery folder set, and in POP-only profiles the default file also holds the default Calendar and Contacts that Outlook shows first. That is why choosing the right default matters beyond this one problem - and why an IMAP or Exchange OST, which mirrors a server, plays by different rules and is not chosen as a POP delivery target.
Two honest causes. First, you have selected an OST file: OSTs belong to their IMAP or Exchange account and cannot be made the profile default in the same way - select a PST instead. Second, the button can stay disabled if the profile itself is damaged; test with a new Outlook profile from Control Panel, Mail, before concluding anything worse. Neither case is fixed by reinstalling Office.
The full fix, shown live in Outlook - the account settings screens, the delivery location and the test mail arriving - is in the video above.
Watch Full VideoThe one whose Inbox you actually read and work in - normally the large, daily-changing file created when the account was set up. Archive files for old years should stay attached for searching but should not hold the default tick, or new mail will start landing in last year archive.
A per-account delivery folder. An individual POP account can be pointed at a specific Inbox, and that setting wins over the profile default. Open Account Settings, select the account on the E-mail tab, and use Change Folder to aim it at the Inbox you want - there is a dedicated guide for that in this series.
No. The default only steers new deliveries from the moment Outlook restarts. Mail already sitting in the wrong file stays there until you select it and drag or Move it to the right Inbox. That is deliberate - Outlook never silently rearranges your existing folders.