From MozillaZine Knowledge Base
- This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).
If you received an alert saying that new messages have arrived but there are no new messages in your Inbox folder and you didn't get an error message, use this check list.
- Make sure the drop down View menu is set to "View all messages. See Disappearing mail for other ways you might overlook a message.
- IMAP accounts support multiple email clients reading messages in the same mailbox. If you read the message using one email client that message will appear as read in the other email clients.
- Thunderbird may have run into a problem downloading a "bad" (malformed) message. There is also a bug that can cause a similar problem (with some POP servers) if there is a pattern of certain control characters in the message body. You can workaround this by using webmail (a browser) to read and then delete the "bad" message. This should free up the logjam and let Thunderbird download any other new messages the next time it checks for new mail.
- If you see a "Building summary file" message (in Thunderbird's status bar) for a long time, or if nothing else listed above seems to be the problem, try this workaround. You might have a badly corrupted folder due to not compacting it often enough.
- You might have to unlock a captcha (“Completely Automated Public Turing test to tell Computers and Humans Apart) using webmail. Some email providers (Gmail, Hotmail and Yahoo do this) lock POP accounts if you make several unsuccessful attempts to login and they suspect the account is being attacked by a 'bot'. When this occurs you need to use a browser to login to webmail and enter the additional data the captcha requests, since Thunderbird has no way to display the captcha.
- Run Enumprocess to see if you have any remnants of a prior firewall still running. For example, Norton doesn't uninstall cleanly and tends to leave behind pieces that may eventually cause problems. If its a Norton/Symantec product you can get a removal tool here.
- If your are using Unified Folders try changing it to All Folders using the ◄► arrows at the top of the folder pane. Several peoples have reported that it "Doesn't always show new messages as they come in, and often thinks there are unread ones when there aren't."
If you're using Gmail see: