Tell Thunderbird not to truncate or wrap lines when sending email

https://help.ubuntu.com/community/Thunderbird#Disable_word_wrapping_in_outgoing_messages

Disable word wrapping in outgoing messages

• Edit -> Preferences -> Advanced -> General -> Config Editor
• Set the preference called "mail.wrap_long_lines" to false

See also: http://kb.mozillazine.org/Plain_text_e-mail_(Thunderbird)#Flowed_format

... mail.wrap_long_lines to control the wrapping of long lines (defaults to true)

Setting mailnews.wraplength to 0 disables wrapping on compose window, but as a side-effect does not wrap sent messages.

I addressed this problem by setting:

mailnews.wraplength 0
mail.compose.wrap_to_window_width true
plain_text.wrap_long_lines true
view_source.wrap_long_lines true

For me, the accepted answer did not show any effect.

As Joost pointed out in his comment, the correct solution is setting

mailnews.wraplength → 0

in the config editor (Edit → Preferences → Advanced → General → Config Editor...).