Correct encoding of carriage return and new line with ofxSmtp / Poco

Hi there,

I'm using the wonderfull addon ofxSmtp to send mail from ofApp
I'm trying to have a content with few lines.
First of all I tried to add \r in my string and add the content this way

 message->addContent(new Poco::Net::StringPartSource(myString, "text/html; charset=ISO-8859-1"),     Poco::Net::MailMessage::ENCODING_7BIT);

but it does not work.
Then I've tried to create a string for every line. Like that :

string myString1 = "....";
string myString2= "....";
message->addContent(new Poco::Net::StringPartSource(myString1, "text/html; charset=ISO-8859-1"), Poco::Net::MailMessage::ENCODING_7BIT);
message->addContent(new Poco::Net::StringPartSource(myString2, "text/html; charset=ISO-8859-1"), Poco::Net::MailMessage::ENCODING_7BIT);

This time it works but I have a strange line between every line in Thunderbird like that.

Capture d’écran 2016-01-13 à 21.58.21.png802x280 37.8 KB

I don't see that in gmail, but it's rendering that way in thunderbird.

I find a page about html formatting issues
https://support.sendgrid.com/hc/en-us/articles/200182068-HTML-Formatting-Issues
but it seems that I have already set it correctly

Is there a way to add a new line in the content without this line ?