What OS and what version of Thunderbird are you using ?

This sounds like a bug reported quite a while back and was fixed.

How was it fixed?

Well if version x had the problem and in version x+1 it has been resolved, the answer to "how was it fixed" is, in practical terms, to install version x+1. That's why you were asked what version you are running.

My own experiences of this bug have been related to address book corruption. Exporting the data, deleting the address book and re-importing the data repaired it for me. But I would hope that the fix referred to by Toad-Hall attended to the underlying reason for the address book being damaged.

I replied to Toad-Hall the OS and version numbers. I did not get a reply that I am aware of. What do you mean by version X=1?

Where? No help to anyone else if you keep them secret.

x = 6 x + 1 = 7

i.e. the next version.

re:I replied to Toad-Hall the OS and version numbers. I did not get a reply that I am aware of.

When you post the info in this question, all who are following this question will get an email. As there is no posting of the information, it would be not possible to repond and it would be useful to know so that you can be offered the best advice. Please tell us what OS and what version of TB you are currently using. This is where the TB version info is located. Help > About thunderbird

Toad-Hall,

Sorry about the error. I am using Windows 8 and TB version 31.7.0

Does your Mailing List have a name which has a space between two or more words eg: Client Group If yes, could you rename the list to say: eg: Client_Group where an underscore links the words.

then retest sending to Mailing list. Please report back on results.

All names are single words.

Do you have a mailing list inside another mailing list (nested)

No. The mailing lists are in an address book. Again, this error just came up. It had been fine in the past.

Yeah great they are in the address book. Now go thought the mailing list. Is there an entry for someone that does not have a valid Someone@somewhere.com mail address or a reference to another mailing list?

No. There are multiple group lists and the issue occurs for all of them. I even created a new one with only me in it and it still occurs. Again, this happen all of a sudden. There is a bug or malfunction in the program. I even updated the software. I would like to uninstall and reinstall Thunderbird but don't know how to save my history and files.