ok, I've spent some time doing various scenarios. I have managed to replicate one scenario which does as you say.

If my contact in the address book:

With name details as Peter Foo email address: peter@somwhere.com where there is an inserted space before the name peter, of course this is not instantly noticeable, but it is as if you had hit the space bar just before typing the email address.

Then when I use the Contacts Sidebar in a Write message, it will add the email address like this: Peter Foo <" peter"@somewhere.com> which of course in invalid.

but in the address book, if highlight the contact, in the section below the email address in blue -if I click this to send an email the email address is printed exacly with the space and it will send as if nothing is wrong.

So i'm asking you to check those email addresses which insert incorrectly via the Contacts sidebar. They only do this for me if there is a hidden inserted space at the start of the email address.

It can also occur if you are copy pasting and inadvertantly insert a space.

so a space is being added before the email address by one of those methods.

The best method is to use one line per email address. Use the 'Contacts Sidebar' to select the various email address/contacts and then click on eg: 'Addto TO ' button or another option.

Thank you. Actually, the list of names is just from a text file. However, the string is so long, that it is separated into multiple lines with <cr><lf>'s. They are creating the double-quotes. Don't really trust address books. Too easy to hack from java.

I have spent an hour today with the tech guys that our firm use and what we have discovered is that there must be a bug in the latest update of Thunderbird as it always worked before but now doesn't. Bottom line is that it will not longer accept multiple email addresses on one line separated by ; as it use to and which is accepted by other email clients. Even without copying / pasting the emails in, even writing each in manually but separating each with the semi-colon it still created this problem. Only way round it that we can see is to paste or write each email address in to a new 'To' field which is really time consuming so I have unfortunately stopped using thunderbird and gone with another. Thanks anyway for your help, maybe Thunderbird will see why this is happening since their last up date and fix the bug as I have been using Thunderbird for over 3 years happily until now.

Since I removed the <cr><lf>'s and have a single long string containing all of the names, there has been no problem. The hassle of attempting to enter the 24+ names in our user group emails would force me to revert back to Micro$lop Outhouse Expresso.

I've just performed some tests where entries are entered into one TO field, instead of the usual method of using one TO field per email address.

Manually type email address, immediately followed by comma then a space then second email address, all sent and received with no issues.

Enter email address, immediately followed by semi-colon, then a space then second email address. Looks like it sent ok, but on checking the email in 'Sent' folder it only shows the first email address, so it effectively removed the second email address. Obviously, I only received one email.

I must admit that I've never used semi-colons before. In the past, I've either sent one email per TO field or used a comma and space as per first test which still works perfectly ok.

See some info here: http://kb.mozillazine.org/Enter_email_addresses

It does say that commas can be used, but not semi- colon. You can enter more than one email address in a header line by separating each address with a comma. Thunderbird does not support using semicolons to do that like Outlook does (section 3.4.6 in RFC822 states the semi-colon has a different function).