The best approach is to use the Inbox as an Inbox for incoming messages and not as a general storage. Keep it low in email numbers and compact on a regular basis. Please do all the following in the order stated and report back on whether you can then download emails. '''1. '''Create folders and organise your emails by moving them into suitably named folders. eg: Family, Friends, Bills, Online Orders, Thunderbird etc. '''2.''' Delete any emails you do not want. '''3'''. Empty the 'Junk' and 'deleted/Trash' folder. * Right click on folder and select eg: 'Empty Junk' Also check the 'Sent' folder as this can also get quite large. Deleting anything you do not want to keep. '''4.''' Then, after doing all of the above, compact all folders to remove the hidden 'marked as deleted' emails. * File > Compact folders. '''5. '''Archive all older emails, say anything older than a couple of months. To do this, set up your Archive Options to store by monthly (If you send and receive several emails each day). Select to keep folder structure. Then Archive emails including the Sent folder. Info on 'Archive Options' here: * https://support.mozilla.org/en-US/kb/archived-messages This will reduce the size of your folders, organise emails and make it less risky. Additional good helpful info: * http://kb.mozillazine.org/Keep_it_working_-_Thunderbird * http://kb.mozillazine.org/Compacting_folders I use an addon to help me know how big folders are getting. * https://addons.mozilla.org/en-us/thunderbird/addon/extra-folder-columns/ I have only the 'Size' column displayed, but it also offers Total emails (also useful) and Unread.