The email client Thunderbird does not necessarily retrieve messages from all of your mail accounts at startup. By default the first mail account will be checked on startup and all mail accounts will be checked every ten minutes. The following user.js tweak changes this behavior in the way that Thunderbird retrieves mail from all accounts at startup as well.
All you need to do is locate the file user.js which should be in your profiles folder. The user.js file does not exist by default and you might need to create it my right-clicking and choosing new, text document and naming it user.js. Once this is done you are ready to add the following lines of code to it.
user_pref(”mail.check_all_imap_folders_for_new”, true);
user_pref(”mail.check_all_pop_folders_for_new”, true);
Once this is done restart thunderbird and mail from all accounts should be automatically received.

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I did exactly as mentioned above, but it doesn’t work for me. :(
here is why it worked for me and not for you. Check your account settings, click on server settings for any account that is not the main account. Activate Check for new messages at startup and Automatically download new messages.
Once you have done that all mail will be checked at startup and downloaded as well..
Works fine. Is it possible to tweak the “Get mail”-button to get new mail for all accounts?
Couldn’t get this fix to work for me on Thunderbird 2.0. Could just be a strange thing on my system only.
However, using the Easy Get Mail Button version 0.32 worked for me. It might work for you, too. https://addons.mozilla.org/en-US/thunderbird/addon/1877
Hey Martin, thanks for the tip. That’s what was wrong in my settings.