Ok, so the way OVH can solve the problem is the same way e.g. Amazon SES solves the problem. 1. They block all outgoing mail (outgoing 25, 465 and 587) and onl

Ok, so the way OVH can solve the problem is the same way e.g. Amazon SES solves the problem. 1. They block all outgoing mail (outgoing 25, 465 and 587) and only open it up once you have submitted a ticket where you decribe for what purpose and how often you want to send mail (if you start off wanting to send a million mails per week they won't open outgoing mail at all). Manually review of those tickets is required so that bad actors can't get through. Difficulty of getting through the review needs to be adjusted upwards till it is too much work for bad actors (and they switch to another provider). 2. Monitor bounce-backs and have a spam reporting form for OVH IP ranges. Start blocking outgoing mail pretty soon if a small number of those arrive. Of course this is a cut-throat business ("email marketing"/spam), so competitors may submit false spam reports... 3. Therefore, for (2) you'll have to keep some records of the mail sent, i.e. you'll require mail to be send via relay... The review and the mail relay costs something, so you make it a monthly/per mail cost. Voilà, you have Amazon SES. Currently they probably block individual instances if they get a lot of complaints about one sending a lot of spam. They can't distinguish legitimate mail from spam because it is all SSL encrypted.