does not sent ALERT mail with smtp due to problem with images

alert mail with embedded images are not sent if you configure observium to send them trough an external smtp server.
email without images are sent correctly.
looking with tcpdump on the mail server it looks like observium does not close the mail with \n.\n so the mta timeout the connection without sending the mail (without the image attach it does send \n.\n and RECOVER mail arrive correctly);

it looks like email are cut at about 20K and that's why it does not close corretly the smtp session.