Today I’ve received email with a line long 6_000px:
As you can see in the styles inspector, the responsible style is: wite-space: pre;.
But then I opened it in Gmail webpage to see how they will cope with this issue and suddenly it’s: wite-space: pre-wrap! 
I can forward you the email if needed.
In any case, this is not exactly my area of expertise, but could Thunderbird fix emails like Gmail somehow? 
Thanks!