Provides an Administrative UI and Developers API for safely updating the mail_system configuration variable.
The 6.x branch also provides a Drupal-6 backport of the Drupal-7 mail system.
The administrative interface is at admin/config/system/mailsystem. A screenshot is available.
variable_get('mail_line_endings', MAIL_LINE_ENDINGS)
MAIL_LINE_ENDINGS
mailsystem_html_to_text()
filter.module
html_to_text.inc