What is a validation SLA or uptime guarantee?
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An SLA (service level agreement) is basically a promise. The vendor's saying: "Our validation API will be working X% of the time, and if it's not, here's what we'll do about it."
For validation vendors, you'll typically see guarantees like 99% or 99.9% uptime. The difference matters. 99% means about 7 hours of downtime per month. 99.9% means about 40 minutes per month. Small sounding, until your real-time signup form can't verify addresses for two hours.
Why this matters: If your ESP has an integration that validates addresses in real time as people sign up, and the validation API goes down, what happens? Some integrations queue the emails for later validation. Others just let them through. Some block the signup entirely. You need to know the vendor's SLA before you commit, because downtime during your peak signup times (Black Friday, product launch, etc.) directly costs you conversions.
Beyond just the percentage, a good SLA also includes response time commitments. For example: "99.9% uptime AND average API response under 200ms." Uptime doesn't mean much if the API is technically online but responding in 10 seconds. That'll gum up your signup flow just as badly.
Check the vendor's public status page before you sign up. Do they publish real uptime numbers? Do they post incident reports when something breaks? That transparency is usually a sign they take reliability seriously. And ask whether their SLA covers your use case. Some vendors have different guarantees for real-time vs. batch validation.
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