What version of Thunderbird? What is your account Type - POP or IMAP? What is your anti-virus software?

I am using Thunderbird Version 128.2.0esr (64 Bit) I am using IMAP I use McAfee anti-virus

You most likely experience the folder corruption problem described in this bug. https://bugzilla.mozilla.org/show_bug.cgi?id=1890230 Try to repair the affected folder(s). Right-click the folder - Properties - Repair Folder <blockquote> I use McAfee anti-virus </blockquote> Another possibility is McAfee causing Thunderbird mail file corruption. These are some generic suggestions to avoid problems with anti-virus software. Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile Don't let your anti-virus software scan incoming and outgoing messages. Don't let your anti-virus software scan attachments. Don't let your anti-virus software intercept your secure connection to the server. Remove any add-ons your anti-virus software may have installed in Thunderbird. Keep it working. https://kb.mozillazine.org/Keep_it_working_-_Thunderbird And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile