BACKGROUND
I am using MS Outlook for personal correspondence on my home computer from the stand alone version of MS Office 2019 (NOT Office 365, not cloud-hosted). I am the only user. I send and receive mail from

• an email address provided by my ISP
• a couple of email addresses associated with one of my domains
• a couple of email addresses associated with my other domain

For many years, these all worked together seamlessly in every version of Outlook that I used. (Version 2019 was the last one I purchased before I retired). I have moved the websites to different servers hosted by different companies several times, but never had multiple data files turn up in Outlook before (with or without messages in them).


PROBLEM
I now have several different data files that serve no purpose and I don't ever remember creating them:

• One is named for the email address issued by my ISP. Apparently it is not being used. Messages that I send and receive with this account appear in the Inbox and Sent Items folders in the main data file. I'd like to remove the unused data file named for this email address safely, without disturbing anything else.
  
• The second is named for an email address that I created but never used. I deleted the account from the host, so it no longer exists as a valid email address, but the data file is still in Outlook and I can't seem to get rid of it.
  
• The third is for my main email address. Email messages that I send from it appear under "Sent" in this data file and I have to drag each one of them to "Sent" in the main data file to keep them with my other correspondence. Paradoxically, incoming messages to this address appear where they should (in the main data file) and not under the data file named for this email address.

BOTTOM LINE
I want to get back what I originally had -- i.e. all email addresses using the same main data file, as they once did -- and to remove all the data files that aren't being used. I want to do this in the simplest and safest possible way. (Preferably, a method that is reversable, if I make a mistake). Any ideas? Thanks.