It is possible that when you select the email, it is being scanned by your anti-virus software and so stops further action - not responding - until the scan is complete. First thing I would try is to disable your anti-virus scanning of emails. It will still scan any attachments you open. If you are using an IMAP mail account, when you view subscribed folders only the header is downloaded. When you select an email then a copy of the folder is downloaded to a temporary folder to faciliate quicker access. Sunscribed Folders that have been synchronised to download a copy to the Thunderbird profile can also impact on performace. So you may experience some delay on occasions. Another reason for slow reponses are folders that are large. You may not have many emails because you deleted some, but did you compact the folder to remove all traces of 'marked as deleted' emails? To reduce the size of folders consider Archiving old emails including the Sent folder. Info on Archiving options: * https://support.mozilla.org/en-US/kb/archived-messages Info on compacting - what it does and why it is important: * http://kb.mozillazine.org/Compacting_folders Good helpful info on maintaining a healthy Thunderbird: * http://kb.mozillazine.org/Keep_it_working_-_Thunderbird