How big is that 'Sent' file? Often people forget that each email is stored in a single file, one written after the other in the order sent. That file gets larger and larger and this can cause issues like slowness etc. Suggest you delete anything not required. Then Right click on folder and select 'Compact' then set up Archive Options - if not already done. * https://support.mozilla.org/en-US/kb/archived-messages Archive anything older than a month. This will reduce the file size.