Outlook is used on one PC with POP3 data retrieval. Messages are only deleted by the email service provider if they are deleted in Outlook. Thunderbird is also used to access the same accounts with IMAP access to the online accounts. All of the messages online have been downloaded by Thunderbird. I am comparing the .pst Outlook files, and the various Thunderbird data files stored by the PC. The Thunderbird data files are much larger than the Outlook files. For example, the largest Outlook .pst file is 2G. But there is a Thunderbird data file that is 8G. Any reason or problem or cleanup suggestion regarding 20G of Outlook .pst data that appears as 30G of Thunderbird locally stored downloaded IMAP data? John