Thanks for the tip, I contacted OVH about this, we’ll see what they say. 200-500 mails per week with DKIM/SPF + official headers and an unsubscribe buttons is not asking them too much though to have it not considered at spam.
Update From OVH = 200 mails per hours ; nice limit for a free mail though 
and to avoid spam here is their suggestion :
- Three or more words in capital letters in the subject line of your email. * No subject line or no text in the email. * The email contains only one large image (> 1MB) and a few words, or many images and little text. * Certain keywords in the subject line or at the beginning of the email: Hello, FREE, BUY, BUYING, etc. * The email contains more than 70% white space (excessive use of the SPACE or ENTER key on the keyboard). * The font used to write the email is too large. * The font color and background color are identical, making the text unreadable. * The link(s) in the email are incorrect. * The URLs are not secure (e.g., declared as https:// when the URL only exists as http://) or link to a hacked or malicious site. * The email contains pornographic or similar terms. * The email contains an executable file (EXE, BAT, PIF, XML, XLSX, or any document with a macro), even if zipped. * Your recipients are listed in BCC only or mainly, even though you are not using a system suitable for mailing lists.