Please follow the steps given below:
A. Open Thunderbird. Right click Inbox. Choose Properties. This will gave you a window containing a button labeled 'Repair Folder'. click this Repair folder option.
B. If step A does not resolve the issue,
Please do the following.
Move all good emails out of 'Inbox' into another suitable folder for organizing. Delete all unwanted emails. When 'Inbox' is empty, right click on 'Inbox' folder and select 'Compact'.
right click on Junk folder and select 'Empty Junk'. right click on 'Junk' folder and select 'Compact'.
right click on 'Deleted/Trash' folder and select 'Empty deleted'.
You do not usually need to empty the Inbox before compacting, but if you suspect any corruption on that folder it is a wise procedure.
C. If not fixed,
Please login into your webmail account and delete the already downloaded emails.
D. Check date and time on the computer and correct it if it is wrong.
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