Yahoo problems

The following two options should workaround the bug in the Yahoo mail system that affect many more than just those with @Yahoo mail addresses.  There is also a Thunderbird bug that has been raised to investigate this issue here that provides more detailed information should you be interested. 

Select options from the three bar menu.
Select the Display | Formatting tab | Advanced button
Set outgoing mail text encoding to Unicode Select the "When possible, use the default text encoding in replies" Click on Ok.

If that does not do it for you you will need to use the config editor to modify the setting mail.strictly_mime to true to force Thunderbird to use 7bit encoding in email.  This is not an optimal setting but is sometimes needed to workaround the various Yahoo issues.