If you have a Pop mail account: Thunderbird can only access the server Inbox and download to pop Inbox. So if the server filters have decided the email is spam and put it into the server Spam folder then Thunderbird will not know about that email. You would need to logon to webmail and see if emails are in server Spam folder. Please note that many server do auto empty the server Spam and Trash folders periodically. If email is in Spam then you need to instruct server that it is not spam, so that email stays in the Inbox and can get downloaded. If you have an Imap Mail account: Then Thunderbird can only display whatever is on the server in the folders you have subscribed to see. Emails cannot not get received onto any server if your mail account has reached it's server quota limit as there is no room. Only when some space is created - emails are deleted - then new mail can be received. This can also effect you sending emails as sent mail cannot get stored. Make sure you have not reached your server quota limits. This is more likely to occur unknowingly if you have a pop mail account. The only thing you can do is access webmail account and delete old emails. Please note if using an imap account then anything you delete via webmail account will also get deleted in an imap account so do put copies of important emails into 'Local Folders' first. It is also possible the server is not accepting any emails from specific senders because those people are blacklisted. Those people can check out if they are blacklisted by using eg: spamhaus - https://check.spamhaus.org/ It's also possible you are downloading those emails but cannot see them - eg: if you have set them up to be ignored: Check this to see you have any emails set up as 'ignored' View > Threads > You should have 'all' selected, but for this test also select 'Ignored threads' Ignored threads would appear and display a red circle in the 'threads' column header. See additional info: https://support.mozilla.org/en-US/kb/ignore-threads