Can you offer some additional info. Please confirm the contact is in a Thunderbird address book.

In the address book, choose a name you know causes the issue. Confirm the exact entries. example: First: has first name eg: Anne - no commas, apostrophies etc, Last: has surname eg: Jones - no commas etc Display: eg: Anne or Anne Jones or Mum etc - no commas etc Email: all lowercase eg; ajones@somewhere.com is there any commas etc

When you 'insert it ino the address bar' please explain step by step, the exact process you are using.

Are you putting one email address per TO field?

Thanks. I have checked the address very carefully - all is as you say.

When inserting into address bar I go to that address book and highlight the person I want to send the email to and then at the bottom of the coloumn I click 'Add to To' - if I have highlighted several names I click 'Add to Bcc'.

Strangely if I go to the address book and see all the addresses and highlight the one I want in the section below it gives the details of the person and the email address in blue -if I click this to send an email the email address is correct with no inverted commas!!

I only put one email address per TO field.

Does this help to identify the problem?

Please test: first check that there is only one instance of that email address/contact. Make sure a duplicate is not lurking in another address book.

In Thunderbird Help > restart with addons disabled

then click on Write and use the 'Contacts Sidebar' to insert the email address as before. Please report back on results.

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 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.

Thanks - I have double checked the email addresses and they all seem OK but I have now deleted them and re-inserted them from my database one by one and they appear to be OK. I will really be able to tell if everything is now working when I send out a mass email later this month.

Thanks very much for your help -it looks as though it is sorted but I don't understand what was wrong with the addresses. I have learned more about thunderbird in the process -thanks

The blank space does not seem obvious.

The most common and likely cause is you copy pasted the email address. This can very easily copy paste a blank space and blank spaces do not show up as an easily detected error. But it is the only scenario I found that produces the issue you had - Contact Sidebar invalid address with specific double quotes around first part - pre @ but the blue link is good.

It does not occur if you have an email address sent to you in an email and you use the left click on email address and select 'add to address book'. It should not occur if hand typed unless an error is made.