Redmine

I have a redmine instance sending emails using a GMail account and I realized that emails were being recognized as spam by gmail. I tried to use sendmail as relay to gmail's smtp, but same result.
Using postfix as relay, I could send email by console (using the 'mail' command), but redmine were still being flag as spam, using the same sendmail relay. So I decided to check 'Plain text mail (no HTML)' and voilá! It worked, mail delivered without spam flag.
There must be something on HTML email headers causing this. Here follows two originals, one plain text (delivered ok) and one html (delivered as spam).

Just ask me if you need more info. I'll keep my instance sending plain text emails for now.