Changed default $eoln from \r\n to \n - mail.ru doesn't display multipart messages properly treating everything as plain text. And \r\n for both headers and body EOLNs have the same effect. Only \r for both headers and body make it work. Gmail is fine either way.
Ability to seamlessly inline <style> into the HTML form of message being sent using inlinestyle/inlinestyle Composer package (not included).