There's a particular issue with Gmail which may be part of your problem. Unlike many email providers (and email clients like Thunderbird) Gmail doesn't use mail folders: it has labels instead. Their idea is that you can have more than one label for the same message - 'family', 'important' or whatever. All incoming mail is labelled 'inbox', obviously. But it also receives the 'all mail' label because that applies to everything in your Gmail account. When Thunderbird retrieves your Gmail messages it takes a 'label = folder' approach. So mail labelled 'inbox' goes into your Thunderbird Inbox. But Thunderbird will also find the same message with the 'all mail' label, so it downloads that as well. The solution is to have Thunderbird '''not''' subscribe to the 'all mail' label. There's guidance here on how to get Gmail to work properly with Thunderbird: https://support.mozilla.org/en-US/kb/thunderbird-and-gmail and a post here about how to manage the same issue with Outlook: http://lifehacker.com/5555291/how-make-gmail-play-nicely-with-your-desktop-email-client