You need to offer information and explain specific issues. It is not necessary to be repetitive, but offering no information is totally useless. Do not use capital letters as this is considered shouting in forum netiquette.

Please tell the forum: what OS you use? What version of thunderbird? Are you talking about one mail account or more? What type of mail accounts do you use?

Please provide the following: Help > Troubleshooting Information click on 'copy text to clipboard' paste info into this forum edit / remove all info on fonts and printers, but nothing else.

Can you say when this occurs, such as on startup or when sending emails ? How many emails are in the Inbox? Do you Archive mail ?

lock-up seems to be gone, so I 'm closing this topic.

But if you're still around, ...start *Windows'* safe mode with networking enabled - win8 http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode - win7 http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode - XP http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx

Still In Windows safe mode, start thunderbird in safe mode - http://support.mozillamessaging.com/en-US/kb/safe-mode

Does problem go away?

If it does not, and you really are crashing, then please create a new support topic and post your crash IDs per https://support.mozillamessaging.com/en-US/kb/mozilla-crash-reporter#w_viewing-crash-reports