"send by email ..." is not going to Thunderbird - Ubuntu

[nat]

Hello,
I was using XnView on a Windows XP to send images out by email, and after ignoring that first prompt "send email" window form, it would redirect the request and open up a blank email in Thunderbird, with the images attached inside the attachment area. It all worked somewhat and was a workaround that I was using to send out images via email -- in a reduced size. But now, I installed XnView MP on an Ubuntu 13.10 OS and if I use the same "send by email..." menu it will not go to Thunderbird after ignoring the form after clicking "OK". Is there a workaround to edit the .ini file so that XnView will open up Thunderbird directly?
Thank you very much!