There are issues within issues within issues here.

First paste of an unknown object from word to Thunderbird. Was it a graphic this stands a chance or an embedded object from something like Microsoft equation editor. Note here that just because you can see it on your system it may not actually appear to the sender, Thunderbird will send pasted graphics, but is that what it was?

Second, once you send mail you loose all control over what comes back. Do you see any sign of where the graphics used to be in the mail?

This is a freelance job, editing engineering papers. Can't find out for sure but an embedded object from MS equation editor is more likely than a graphic. I can see the equation when I send the email; on return, a mostly empty box appears, with a small gray image that looks like a page ripped horizontally.

Source document for me is Word 2010, but original document may not be, and object could have been copied from that original.

Thanks.

Sounds like a broken embedded object link.

1. Is it broken in your 'Sent' saved email?
2. if you compose email then save as Draft, then close original and open the Draft using 'Edit'..is it broken - torn graphic image ?
3. Do you know if the recipient can read the email ok - all info is clearly visible in their received email?
4. When you say 'on return, a mostly empty box appears,' are you refering to a 'Reply' from the recipient which includes your original message with the embedded object link?

Some other options in Write new message: If typing a specific Maths formula:

• Insert > Mathematical formula
• Type formula
• click on Insert

If a saved image:

• Insert > Image
• click on 'choose file'
• select the file and click on 'Open'
• select : 'Attach this image to the message'
• Click on OK

Write in a eg: MS Word doc and attach to email.