I have email account setup with IMAP and local folder synchronization. Recently I moved my profile to a new machine and when it starts up it keeps re downloading the messages in the local folder. I have over 10 GB of emails and it is not ideal if it re downloads all these messages from the IMAP server all over again. Is there way to force Thunderbird to download messages that are not in the local folder and reuse the already downloaded copy?
2 Answers
In Thunderbird (depending on your version), look for an icon with three stacked horizontal bars (Application Menu). Click on it, then go to the following: Options... (click the dropmarker) > Account Settings... > Synchronization & Storage > Message Synchronizing > checkmark the "Keep messages for this account on this computer" > Advanced... > checkmark any folders you wish to cache messages for offline use. Make sure you have enough available hard drive space.
Caching messages offline will store in your hard drive a copy of all messages. All messages will be downloaded the first time in the folders you select for offline use. Then Thunderbird will download only new messages after that.
I believe you might have figured it out since the question was asked 6 years ago, but adding the solution so that it can help others with same problem.
I had to restart thunderbird(68.7.0) to let it know the messages were already downloaded.
Application menu(three stacked horizontal bars in top right corner) -> Help -> Troubleshooting Information -> Profiles(click about:profiles) -> Restart normally
