IMAP shows a remote view of folders on the server. Emails are stored on the server. You subscribe to see any or all of those folders. Headers are downloaded and when you select to view an email, it is retrieved from the server and stored in a temporary cache to faciltate viewing. These emails are not stored in thunderbird and cannot be viewed if in offline mode or no internet connectivity.

You can synchronise folders for offline use. This downloads a copy of emails to the folder and is stored in the Thunderbird profile folder. However, those folders are constantly being synchronised with server, so cannot be relied upon as a backup. Here is information on setting up synchronising:

• https://support.mozilla.org/en-US/kb/imap-synchronization

'File' > 'Offline' > 'Download/Synch' will get an up to date copy into Thunderbird for using in offline mode.

If you need to get a backup copy of emails, then you need to do this synchronising and then going into offline mode before creating a backup. In this offline mode, you can also use 'Copy to' a folder in 'Local Folders' account, which can be useful if you need to delete emails off server to create space, but do not want to lose a copy.

I would recommend that you create regular backups. If you delete an email in webmail, using phone or via ~Thunderbird, the email is deleted off the server. EG: emails deleted by phone or webmail will disappear in Thunderbird.

'Tools' > 'Account Settings' > 'Server Settings' for the imap mail account Here you can select to make Thunderbird check for new messages when TB starts up. You can also select to check for new messages every eg: 10 minutes.

However, if the server supports IDLE command, then these settings may be overwritten by the IDLE command. The IDLE command is a extension to the IMAP protocol that lets the IMAP server notify Thunderbird when there is new mail by sending a command over the existing TCP-IP connection. If your IMAP server supports that you don't have to manually check for new mail or have Thunderbird poll for new mail every x minutes.

You can switch off IDLE, but it would be more convenient to leave it working as you would get a m ore instant notice of new messages. it will still connect with server upon startup: 'Tools' > 'Account Settings' > 'Server Settings' for the imap mail account click on 'Advanced ' button uncheck: 'Use IDLE command if the server supports it' click on all OK's

Backup info:

• http://kb.mozillazine.org/Profile_backup
• http://kb.mozillazine.org/IMAP_backup

additional link with specific references to gmail:

• https://support.mozilla.org/en-US/kb/thunderbird-and-gmail
• http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite
• https://support.google.com/mail/answer/78892?hl=en

Thanks so much...very clear explanation and (hopefully!) solution! Just as an FYI, the Google answer has some of the instructions wrong...I suspect that' s because it was written with an older version of T'bird in mind. I was able to find the code links by simply scrolling, however, and looking for key words.