If your email provider's server or your local computer's storage is full, new messages cannot be saved.
By default, Thunderbird might only download headers or a limited number of recent messages to save disk space. why does thunderbird not download all messages
The "Global Database" ( global-messages-db.sqlite ) or folder index files ( .msf ) can become corrupted, preventing the client from correctly recognizing and displaying new mail. If your email provider's server or your local
Antivirus or firewalls may block Thunderbird’s connection to the mail server, especially if they use deep packet inspection for email. Enable Full Synchronization (IMAP)
If you use IMAP, ensure Thunderbird is set to download complete copies of your messages for offline use:
For POP3 accounts, Thunderbird might ignore messages already marked as "read" on the server. How to Fix Thunderbird Synchronization Issues 1. Enable Full Synchronization (IMAP)