You not mention whether you have a Pop or IMAP mail account. '''If IMAP''' Providing you can still access the old server and emails are still there. Synchronise your mail folders to get a copy of the server folders onto your computer. Info on synchronising subscribed folders: * https://support.mozilla.org/en-US/kb/imap-synchronization#w_configuring-synchronization-and-disk-space-usage Then stop the synchronising. Go into Offline mode. In Offline mode, move emails into Local Folders. Emails in Local Folders will be stored on your computer. When all emails are safely in Local folders, then you can delete the mail account. Also in Offline mode you could create a backup of those emails. Use this addon: * https://freeshell.de//~kaosmos/mboximport-en.html How to install addon: * https://support.mozilla.org/en-US/kb/add-ons-and-extensions-faq#w_how-do-i-install-an-add-on To use, right click on folder to see ImportExporttool options See image below. Note: you can also use this tool to import backedup emails into Local folders as another method of getting them into Local Folders. '''If Pop:''' The emails are already on your computer. So move emails into Local Folders and when you are certain that you can see all emails in Local Folders, then you can delete the Pop mail account.