Why this is happening but as Vista has both it's IE AND Mail Clients Divorced from the Kernel I would imagine that Thunderbird is equally divorced. There may also be some size limits set as Default by the OS which will not allow to be sent for Security Reasons.
From this prospective you are effectively a Beta Tester for a new product which no one is quite sure how it works. As this is a new computer that came loaded with Vista and you didn't mention which version so that only makes it harder you really only have 2 options neither of which I would hold out much hope on.
[b]1[/b] You can ring the maker of this system and ask for technical help but as they will know next to nothing about how Vista Works and that you have loaded Thunderbird they will blame Thunderbird as the problem and that's about as far as I think that you'll get there.
[b]2[/b] You can post a question to the Windows Vista Forum located here
http://tinyurl.com/yt8adb
Though from what I've seen of this place they are devoted M$ Junkies who if they bother answering will in all likely hood insist that Thunderbird isn't a recommended part of Vista so they don't know how it works and will insist that you use the Mail Client that comes shipped with Vista which probably will not work either. From what I've seen posted at this site it seems to be end users attempting to give advice to other end users without any intervention by M$ Staff so that may not be of any help to you but it's worth a try.
You could also contact Mozilla who make Thunderbird and ask them if there are any [b]Known Issues[/b] with Thunderbird and your version of Vista that will probably be your best option though I wouldn't be expecting a fast response.
http://tinyurl.com/yt8adb
Lastly and this is by every means the last option you could ring M$ Technical Support and ask. However knowing M$ Tech Support they will say that they don't support third Party Programs so contact the supplier of the program for assistance and then give you a nice bill for that useless information.
Col
