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@gno All right, so I’ve got more informatin now.
I’ve contacted OVH, nothing was wrong with the IP. They couldn’t do anything. I also tried sending mails via mandrillapp (so with the same domain!!). That works (but I don’t want that) ^ This may be an indication for something we’re missing here… Any idea? I’m now also sending mail via TLS instead of plain, because Google is now checking that as well. Still no solution tho.
Mail from header that goes through: http://upl.drc.ms/1O5s7W8b9L.txt
Mail from header that gets blocked: http://upl.drc.ms/2G843s4g0h.txt