Handles the mail_settings attribute in SendgridBackend._build_sg_mail.
Note: if sandbox_mode is set in the supplied settings, its value is
overwritten with the default defined in this library's documentation to
preserve as much existing behavior as possible. But this can be changed if
desired to allow the user to provide a per-message sandbox_mode value in the
supplied MailSettings.
Handles the mail_settings attribute in SendgridBackend._build_sg_mail.
Note: if sandbox_mode is set in the supplied settings, its value is overwritten with the default defined in this library's documentation to preserve as much existing behavior as possible. But this can be changed if desired to allow the user to provide a per-message sandbox_mode value in the supplied MailSettings.