I have problems too with this update, in my case, I delete my account and create again, but is a imap account, then I don't losing anything, my personal address remained intact.

UPDATE: There are a lot of reports of this issue on Bugzilla (which I probably should have checked before posting here). A fix appears to come in Thunderbird v. 32 (though several who filed the same bug asked for an expedited fix because of its importance). In the meantime, here are the workarounds (I haven't tried them) from Bugzilla: "Confirmed workarounds:

• While composing an email open the address book and select the list you are trying to send to, highlight all the names in the list and drag them to the To: box. This uses your existing data without modifying it.
• Replacing the blanks " " between the words in such lists' descriptions with an underscore "_". This requires modifying your mailing list(s) description(s)."

Yes, this has been reported as Bug #1060901, many other duplicate bugs are being reported as such. Other people have asked about it here too. If you have an account on Bugzilla, please consider voting for that issue.

Several other people have sent in the same support request as you, noting this happened after they upgraded to version 31.1.

The exact error message is: XXXX is not a valid e-mail address because it is not of the form user@host. You must correct it before sending the e-mail.

This happens in Thunderbird 31.1.0 when your mailing list description includes several words separated by spaces.

I was the one who confirmed the workarounds with other users and commented on that bug report.

Another, probably more sensible approach, specially if email is critical to you, is to revert to a previous Thunderbird version and disable automatic updates (Windows).

In Windows Thunderbird updates are automatic by default. To avoid similar situations in the future, you could change that and decide to have a more conservative approach to upgrades.

Before trying anything, make a backup of your current Thunderbird profile.

You can uninstall the current Thunderbird version via Control Panel > Programs and Features and reinstall a previous version, download it from here:

• Version 31.0
• Version 24.8

Click on the type for your OS. eg: If using Windows select 'win32' then select the language eg: en-GB = English british then download the .exe file

You can customize automatic updates behavior, however, it's under Tools > Options > Advanced > Update.

My suggestion would be to set it to Check for updates, but let me choose whether to install them., specially if you depend on email for critical / important business or personal matters.

I had a problem sending mail to a list I had used successfully in Eudora. Imported from Eudora and it wasn't accepted in Thunderbird with the error message quoted above. My list name was 2 words. I deleted the space between the 2 words, and can now send mail to that list successfullly.