There is a previous thread for this question which has been archived. If compacting folders does not work, here is another solution. Short answer: empty your inbox by moving all of the emails in it into subfolders. If your inbox has a lot of emails, it is best to divide them up into several subfolders. After the inbox is empty, right click on the inbox folder name and chose compact. This should resolve the problem. If the inbox is not completely empty, compacting it may not resolve the issue. If it doesn't, here is a more detailed explanation on how to accomplish the same thing. When Thunderbird is open, go to Accounts, View Settings for this account, Server settings, and then note the location of the local directory which holds your downloaded emails. Navigate to that folder in Windows explorer. This may be a hidden folder. If you can't find the local directory, you need to modify folder options. Open Folder Options by clicking the Start button, clicking Control Panel, clicking Appearance and Personalization (or Folder Options, View tab., Advanced settings, click Show hidden files, folders, and drives, and then click OK. When this folder is visible, you can see your Thunderbird download folders (inbox, drafts, sent, trash). If you have created more download folders, you will see them listed also. Each folder has an index file with a .msf extension. These msf files keep track of what is in the folder. When Thunderbird is closed, msf files can be deleted without harm, as Thunderbird will rebuild the file when it reopens. Note the size of your inbox folder and the inbox.msf file. Chances are your inbox folder will be quite large, and the msf file a much smaller percentage of the inbox foldere. One reason for the "There is not enough disk space to download new messages" error message is that the inbox folder has grown too large. Deleting the inbox.msf file will not solve this issue. What you must do is create one or more new folders in Thunderbird by right clicking on the account name (i.e., fred@gmail.com) and select New folder. Name the new folder in the popup window ("inbox 1" or whatever you want to call it besides "inbox" to be created as a subfolder of your account name (fred@gmail.com). You will see "inbox 1" folder listed in the list of folders under this account. You can create as many subfolders as you like. Next, click on your inbox folder and highlight the e-mails you want to move to your new subfolders by clicking on an e-mail in your inbox and then holding down the shift key while pressing the up or down arrow or the page up or page down key. After you have highlighted the e-mails you want to move, then click on the Message tab at the top of Thunderbird, then "Move to," and then select the subfolder you want the messages to be moved to. After you move some e-mails, you may be asked if you want the folders compacted, and you can say yes. Complete this process until no more messages remain in your inbox. This is very important, because you will be deleting the inbox.msf file and the inbox folder next. Before you do this, make sure the inbox is completely empty so you do not lose any emails! When your inbox is completely empty of e-mails, close Thunderbird and go back to your local profiles folder (which may be something like C:\Users\Fred\AppData\Roaming\Thunderbird\Profiles\fsf7f4fv.default\Mail\inbound. gmail . com Notice that the size of the inbox file has not changed at all or has not changed significantly even though there are no longer any emails in it! This is part of the problem with the inability to download new emails. You may now safely delete both the inbox folder and the inbox.msf files. When you reopen Thunderbird, you will see a new inbox folder and inbox.msf file with a very small size. You can now download new e-mails. If you want to have a backup just in case anything goes wrong with this process, once you have emptied your inbox, delete the inbox.msf file and then rename your inbox folder to something else like inbox-old. This shouldn't be necessary, as you can see all the emails you transferred in the new fodlers. I have done this process many times and never lost an e-mail. The process should be similar with a Mac, but I have not performed this process with a Mac OS, so I can't guarantee that it would work the same. If you try it on a Mac, just make a backup of your inbox folder first . Good luck! This isn't as complicated as it sounds!