Welcome to Sunday Funday: AI vs AI — a weekly experiment on the Engagor blog.
Here's the concept: every Sunday, we take two AI agents and give them an email deliverability topic to debate. No script. No human editing the conversation. We just hit "go" and see what happens.
Think of it like a sparring match between two email nerds. Except they never get tired, never lose patience, and occasionally need a veteran to step in and settle the argument.
This is Episode 3. This week's debate: should you run a multi-ESP setup for redundancy and resilience, or is the operational complexity not worth it? Quinn says spread the risk. Bob says you're creating more problems than you solve.
Got a topic you want our AI agents to tackle next week? Drop it in the comments or DM us on LinkedIn.
This Week's Topic
"Should you run a multi-ESP setup, or stay with a single provider?"
The Conversation
Alright Bob, settle this for me — I've got a client running everything through SendGrid and they're talking about adding Mailgun as a second ESP. I told them it's a no-brainer. Spread the risk. You don't put all your eggs in one basket, right?
Honestly? That advice is going to create more problems than it solves. A second ESP means a second IP reputation to manage, a second domain authentication setup, split engagement data, and double the troubleshooting surface. For what? The illusion of redundancy?
Illusion? Tell that to the companies that got burned when their sole ESP had an infrastructure outage and they couldn't send for 12 hours. Or when one provider's IP range lands on a blocklist and suddenly your entire program is dead. Having a second ESP isn't an illusion — it's an insurance policy.
And how often does that actually happen? Once a year, maybe? Meanwhile, you're paying for two platforms 365 days a year, splitting your sending volume across two IP pools — which means slower reputation building on both — and your deliverability team has to monitor two dashboards, two sets of bounce logs, and two completely different event formats. You're trading a rare catastrophic risk for constant operational pain.
That's a cost argument, not a deliverability argument. And it falls apart at scale. When you're sending millions per month, a multi-ESP setup lets you route promotional through one provider and transactional through another based on their strengths. SparkPost for bulk throughput, SES for transactional reliability. Best tool for the job.
Now you're making my point for me. You just described a setup where your promotional and transactional emails build reputation on completely separate IP pools with completely separate sending domains. So when Gmail starts throttling your promotional sends on ESP A, you have zero positive signal from your transactional volume on ESP B to offset it. You've fragmented your own reputation.
That's not how mailbox provider filtering works anymore and you know it. Gmail and Microsoft are evaluating sender reputation at the domain level, not just IP level. Your d= domain in DKIM is carrying more weight than your IP address in 2026. So splitting across ESPs doesn't fragment your reputation — it consolidates it under your domain.
If that were fully true, nobody would bother with IP warming anymore. The reality is it's both — domain reputation AND IP reputation, weighted differently per mailbox provider. And when something goes wrong on one ESP, good luck doing root cause analysis when your data is scattered across two platforms with different event schemas, different timestamp formats, and different bounce categorizations. "Was it the content? The IP? The segment? Which ESP was this on again?" You've just made every deliverability investigation twice as hard.
That's a tooling problem, not an architecture problem. If your team can't unify data across two ESPs, that's a gap in your monitoring stack, not a reason to avoid multi-ESP. The companies doing this well have a unified analytics layer sitting on top of their ESPs.
Oh, so now the recommendation is: add a second ESP, AND build or buy a unified analytics platform to make sense of the data you just fragmented? You're stacking costs and complexity to solve a problem you created by splitting in the first place.
You're stacking resilience. One ESP goes down, the other keeps sending. One IP range gets blocklisted, you failover. One provider changes their pricing, you have leverage to negotiate. That's not complexity for complexity's sake — that's operational maturity.
Resilience assumes you can actually failover cleanly. In practice, most multi-ESP setups I've seen are not hot-hot. They're running different segments through different providers with no real failover capability. The "backup ESP" has cold IPs, no recent sending reputation, and when you actually need it in an emergency, you're ramping it from zero. That's not an insurance policy — that's a liability.
Time out, both of you. You're arguing absolutes and the answer — as usual — is "it depends."
Bob is right that multi-ESP adds real operational complexity. Fragmented data, split reputation signals, and double the monitoring burden are not theoretical concerns — they're the daily reality for most teams that go multi-ESP without the right infrastructure. I've seen more companies hurt their deliverability by poorly managing two ESPs than by having a single point of failure with one.
Quinn is right that single-ESP concentration risk is real. At scale — we're talking millions per month across multiple brands or business units — relying on one provider is genuinely risky. Outages happen. Blocklist events happen. Pricing leverage matters.
Here's the actual framework:
Stay single-ESP if: You're under 1M sends/month, you have a small team, and you don't have unified cross-ESP monitoring in place. The complexity cost will outweigh the redundancy benefit.
Go multi-ESP if: You're at scale, you have distinct use cases (transactional vs. promotional vs. triggered), AND — this is the critical part — you have the tooling to give you a single pane of glass across all providers. Without unified visibility into delivery, engagement, and bounce patterns across every ESP, you're flying blind on half your program.
The inconvenient truth? Multi-ESP is architecturally sound but operationally brutal without the right monitoring layer. The decision isn't really "one ESP or two?" — it's "do I have the infrastructure to actually manage multiple sending channels as one coherent program?"
Fair enough — I'll concede that telling someone to just "add a second ESP" without addressing the visibility gap is irresponsible advice. The architecture is only as good as your ability to monitor it.
And I'll admit that at scale, single-ESP is a real concentration risk. I just hate seeing mid-market companies cargo-cult the multi-ESP playbook from enterprises without having the operational maturity to pull it off.
Agreed. The tooling has to come first, not after the fact.
On that, we finally agree.
Key Takeaways
What the AIs figured out:
- Multi-ESP adds real operational complexity — fragmented data, split reputation signals, double the monitoring burden, and different event schemas make every investigation harder
- Single-ESP concentration risk is real at scale — outages, blocklist events, and pricing leverage all matter when you're sending millions per month
- Domain reputation matters more than IP reputation in 2026 — but IP reputation still plays a role, and splitting volume slows reputation building on both pools
- Most "backup ESPs" are actually liabilities — cold IPs with no recent sending reputation can't actually failover cleanly in an emergency
- Stay single-ESP under 1M sends/month with a small team — the complexity cost outweighs the redundancy benefit without the right infrastructure
- Go multi-ESP at scale with unified monitoring — but only if you have a single pane of glass across all providers for delivery, engagement, and bounce data
- The real question isn't "one ESP or two?" — it's "do I have the infrastructure to manage multiple sending channels as one coherent program?"
That's a wrap on Episode 3 of Sunday Funday: AI vs AI.
Same time next Sunday. Different topic. Same two AIs who definitely don't need coffee breaks.
Got a topic you want Quinn and Bob to tackle next? Drop it in the comments or DM us on LinkedIn. We're taking requests.
See you next Sunday.
