You should ask this in an Outlook support forum. We help out with the Thunderbird email client here.

I had previously phoned the Outlook help line and they told me it was not their problem but was a problem with Mozilla

PLease clarify.

YouDanegan said

This problem only occurs with a particular client, which is a company. When saving an email address from this company, Outlook adds quotes before and after the name, e.g. "jsmith@abc-def.com" As a result, the email is always returned as undeliverable. Even when I purge the address from the address book and type in the address correctly, Outlook still adds these quotation marks. How can I get Outlook to stop doing this? It is a major client and a major irritation to them and me.

If Outlook does that, talk to them. If Thunderbird does that talk to us.

How do I know who's doing it?

You keep saying 'OUTLOOK', which is a Microsoft product. We are a board that deals with 'THUNDERBIRD', a Mozilla product.

They are two completely separate competing mail clients.

Would you ask FORD to solve a problem with your HONDA?

Could this be a misunderstanding? You have Tbird installed and uses Tbird (not outlook, the program) to access an account on outlook.com ?

If so: Check if you have this address in two different address-books with different spellings. Esp check Collected Addresses Look for an extra space or comma.

An address should look like

Tradera <member-to-member.noreply@tradera.com> 

just a space between name (Tradera) and < No other spaces allowed.