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Thanks for your reply. I tried with the html and text format. All I did in terms of editing was press the enter key to go to nex line when I wanted a new line, eg
John Smith
The Example Company
12, Instance road
and so on. What I get in the email when I right-click and add signature is John Smith The Example Company 12, Instance road
I'm no expert but I guess there must be a simple way of fixing it. Thanks

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I did exactly the same and it works for me...
Does it work without add-signature?
Add-signature just adds the signature as text or html without changing it, it uses the functions included with thunderbird. So I don't really know what or where to fix :-)
Maybe it's a problem with your thunderbird version or installation.