Hello David B_UK
Welcome to Microsoft Community.
I understand that you want to keep old emails from your canceled ISP mailbox account and that you keep the .pst file under the account.
What needs to be confirmed is, did you originally use the POP protocol to configure your ISP email account? I.e., are the contents of the .pst file complete? If not, it is recommended that you re-export a new pst backup file for the emails under your account.
Import and export Outlook email, contacts, and calendar - Microsoft Support
If you want to keep the original configuration content, it is recommended that you either create a new Outlook profile and open your PST data file under the new profile or configure another new account.
Create an Outlook profile - Microsoft Support
Then, click on File - Open & Export in Outlook and use the Open Outlook Data File feature to open your PST file and then view the contents under that data file.
After ensuring that the contents are error-free, you can keep your PST data file and freely choose whether to delete the original account that reported the error.
I hope this information has been helpful to you. If you have any questions, please let me know and I will be happy to help.
Sincere greetings
Forry.F - MSFT | Microsoft Community Support Specialist
Hi Forry.F,
Thank you for your prompt response.
After checking again and addressing your question, I realised that the original ISP account uses IMAP/SMTP, which gives a file extension of .OST, rather than the .PST I initially mentioned. I assume that the advice provided may not apply in this case, and we might need to explore an alternative solution.
One option I was considering was to create a new data file with a .PST extension, replicating the same folder structure as the original file, and transferring the contents to the new .PST file. However, this method seems a bit cumbersome, and I'm hoping we can find a more efficient way to stop synchronisation with the cancelled ISP mailbox account.
Please advise.
Many thanks David B_UK