Check your Account Settings: Right click on imap mail account and select 'Settings' see info at this link. https://support.mozilla.org/en-US/kb/imap-synchronization#w_configuring-synchronization-and-disk-space-usage Select the option to 'keep messages for this account on computer'. Settings for offline use also here: Menu icon > Options > Options > Advanced > 'Network & Disc Space' tab click on 'Offline' button to see options. Good info: http://kb.mozillazine.org/Offline_folders To go offline: Menu icon > File > Offline > choose option Quick way: In bottom 'Status Bar', click on the 'two blue screen icon' to go offline. click on 'Two black screen icon' to go to 'online' mode.