How do I stop Thunderbird mail from truncating all my incoming mail?
This may be happening if you have enabled "To save disk space do not download". If so, disable it.
Thunderbird -> Tools -> Account settings -> go to the particular account (if you have more than one) -> Disk Space -> look at the right panel if there is a checkmark below "To save disk space, do not download" (in front of "Messages larger than...")
On Mac: Thunderbird -> Edit -> Properties ....