Some incoming emails do not appear in my Thunderbird Inbox. The mail environment is as follows. Server: Microsoft Exchange Mailer: Thunderbird Receiving protocol: POP3 They are not in the spam folder. This problem is occurring with some emails from a specific domain. Several people are experiencing the same problem. Also, Outlook on the Web and IMAP can be used to view the messages. Is there a possible reason why some emails are not showing up in POP3 and what can be done about it?