I have been running mail infrastructure for twelve years and I have watched every major provider play the SLA game. For what it's worth, the entire concept is a marketing fiction designed to sound rigorous while costing providers almost nothing.
Here is the math nobody talks about. A 99.9% annual SLA against a $20/month plan. The maximum credit is typically one month. Expected downtime per year: 8.76 hours. Probability of triggering full credit: roughly 10% based on actual provider behavior. Expected value: 0.10 × $20 = $2.40 per year.
That is what your peace of mind is worth. Two dollars and forty cents. The SLA is not a promise. It is a cheap insurance policy with a deductible measured in reputation.
SPF, DKIM, DMARC — holy trinity ✉️