Although there are many reasons why this error message might appear, one of the reasons is that the mail account giving the error shares the same inbox as another mail account. In this case it could most likely mean that the Message Store Type is different for each mail account.
To check the message store type click on Tools->Account Settings
Then click on Server Settings under the specified mail accounts and check the Message Storage Type for each.
Should the Message Storage Types differ for what ever reason change them to be the same.
Follow the steps below to change the Message Storage Type for a email account.
- Select Tools->Options
- Click on the Advanced Tab and then on Config Editor
- Click on I accept the risk!
- Search for mail.serverDefaultStoreContractID and change the Message Storage Type to the Message Storage Type of the working mail account.
MBOX = @mozilla.org/msgstore/berkeleystore;1
MailDir = @mozilla.org/msgstore/maildirstore;1
- Restart Thunderbird.
- Remove and re-add the working email account.
Image Credits: Photo by Isis França on Unsplash.