We highly discourage enabling open tracking as it's just a false positive and increase the chances of landing in spam. it's a pixel injection in the email body and most ESPs filter those as part of spam detection (most of the share to open rates)
Focus on reply rates only
there was a recent ban from MS on some of the resellers. we've been getting some inbound/referrals for the same. not sure the exact cause but heard the damage was really big - 100s of thousands tenants affected.