About Tesco Bank
Tesco Bank launched in 1997 to bring financial services to the customers of Britain's largest supermarket. It serves millions of customers across credit cards, loans, and savings, with a CRM operation built around direct-to-customer communication at scale.
Note: Tesco Bank is a trading name of Barclays Bank UK PLC.
The Challenge: The Emails Were Being Delivered. They Just Weren't Arriving
Tesco Bank was ramping up its CRM program when the emails stopped landing.
The first sign was the oldest one in the book: someone on the leadership team wasn't receiving them. Seedlist members weren't either. On their own, either could have been a fluke. Together with two data points, they weren't.
Delivery rates looked fine. Near 100%, in fact, which is the number most senders check first and then stop checking. But engagement was sliding, unique opens drifting down from 35%, and block rates had jumped.
Read together; those four signals describe a specific failure. The messages were making successful server-to-server delivery, passing from Optimove to Microsoft exactly as intended. They were being accepted. They were simply being accepted into spam folders, where nobody goes to look. Tesco Bank was blocked by Microsoft, and the metric most likely to have caught it was the one metric that looked healthy.
For a bank, that is not a marketing inconvenience. Tesco Bank is regulated, and it is required to make reasonable efforts to reach its customers. Every message that fails to land pushes volume back onto direct mail, at cost, against a paperless commitment. And commercially, the arithmetic is simple: if you cannot reach a customer, you cannot engage them, and you cannot change their behavior.
"Every time you don't land in the inbox, you're missing an opportunity, and you should see it as a commercial problem."
Lucy Wigglesworth, Head of CRM, Tesco Bank
The Solution
Analysis: Find the Problem Before Touching the Fix
Optimove started where it starts with any deliverability case: a full audit before a single change. The instinct under pressure is to act. The discipline is to diagnose.
The team pulled data from both SendGrid and Optimove for the window when performance first slipped, then read it against best practice across frequency, cadence, target groups, and segmentation. The goal was a 360-degree view of the sending program, not a single suspect.
Two things came into focus. First, by looking at ISPs individually, the team could isolate where the damage was. The issue was pinpointed to Microsoft servers. It was not a problem across every mailbox provider. That distinction matters, because a Microsoft-specific block and a global reputation problem call for different fixes.
Second, the SMTP error data told a story the delivery rate had hidden. Mailbox providers return a reason when they refuse a message, and a large share of Tesco Bank's blocks came back as "mailbox full." A full mailbox is not a picky customer. It is a dead address, so overloaded it is receiving nothing from anyone. These were not people declining to engage. They were addresses that no longer belonged to real, active customers, and they were quietly dragging the whole program's reputation down.
That reframed the problem. This was not only a fight with Microsoft's filters. It was a list that had aged, and a reputation paying the price for it.
Reset and Protect
The fixes fell into two groups: repair the immediate damage, then change the sending behavior that had caused it.
To repair it, Optimove filed support tickets with Microsoft to reset Tesco Bank's sender reputation and whitelisted all eight IP addresses across the service and promotional accounts. Whitelisting is not always an option, and rarely in the US, but in the EU some global providers will work with a sender directly, and that route was open here.
To protect it, the team went after the dead weight on the list. They added suppressions across the global target groups in the promotional account, screening out subscribers who had gone non-engaged or lapsed; the exact profiles most likely to be full mailboxes or hard blocks. Fewer bad addresses in every send meant fewer refusals, and a reputation that could recover instead of resetting into the same wall.
One practice outlasted the crisis. On a rolling 30-day basis, Tesco Bank's team exports blocked addresses from SendGrid and uploads them to Optimove as a suppression file, a standing hygiene routine they still run today. It is unglamorous, and it is the single most consequential thing a sender can do.
Not every recommendation was needed. Third-party list validation was on the table but held back, since the 30-day export was already doing the work. DMARC was suggested but already in place on the root domain. One-click unsubscribe was already handled inside Optimove. Where Microsoft's automated spam classifier, Symantec, had thrown false positives, Optimove used its direct relationships with these providers to request human review of messages from before and after the drop. That let a person correct what the machine had gotten wrong.
Results: Back in the Inbox
The fix worked. Tesco Bank's emails returned to the inbox, and customers began receiving them again. Inbox placement recovered, and the sender's reputation that had taken the damage was rebuilt to excellent.
Some of the repair became permanent. The rolling 30-day export of blocked addresses, started during remediation, is still running today, a standing hygiene habit rather than a one-time cleanup.
The change in thinking: The most lasting result was not technical. Having been forced to react to a crisis, Tesco Bank stopped treating deliverability as something to address when it breaks and started treating it as a cornerstone of CRM strategy. It is now owned in-house, prioritized, measured month over month, and reported to leadership, not just when there is a problem but continuously.
"We have the skill set in-house which, coupled with the expertise we can leverage with Optimove, means we know what we're doing. And while we can't always guarantee we won't hit another problem, if we do, we know what to do."
Lucy Wigglesworth, Head of CRM, Tesco Bank
The Bottom Line
You can build a sophisticated email program, personalized, AI-driven, carefully tuned, and none of it matters if the message never reaches the inbox. As Lucy Wigglesworth put it, there is little point building the sophisticated things on top of shaky foundations.
Deliverability is the foundation, and it behaves less like a project than like maintenance. You service a car before it breaks down, not after. You keep the annual checkup rather than waiting for symptoms. Email is no different: the senders who stay in the inbox are the ones who treat deliverability as ongoing upkeep, not emergency repair.
For Tesco Bank, the crisis was the thing that made that obvious. The lasting win was not simply getting unblocked. It was learning to never be caught the same way again.
See how Optimove helps CRM teams diagnose deliverability issues, protect sender reputation, and keep customer communications reaching the inbox. Request a demo.
