Merge tag
A merge tag is a placeholder in an email template that gets replaced with subscriber-specific data at send time. When merge tags break, your email sends with blank fields or raw code visible to the customer.
Definition
What a merge tag is
A merge tag is a variable in an email template that pulls in data from your customer record or product database when the email is sent. Common examples: first name in the greeting, order number in a confirmation, tracking URL in a shipping update, account balance in a statement.
The tag sits in the template as a placeholder, usually wrapped in curly braces, square brackets, or percent signs depending on the platform's templating language. At send time, the platform looks up the corresponding field for the subscriber and swaps the placeholder for the real value. If the lookup succeeds, the customer sees personalised content. If it fails, the customer sees a blank space, the word "null", or the raw tag itself.
Failure modes
How merge tags break
Null or missing data
The most common breakage. The field exists in your schema but has no value for this subscriber. The template renders with a blank or the platform's fallback string.
Field rename or schema change
Your CRM or CDP renames a field, but the template still references the old name. The tag no longer resolves and the raw placeholder appears in the sent email.
Syntax error in the tag
Liquid, Handlebars, and other templating languages have strict syntax. A missing closing brace, a typo in the field name, or an incorrect filter breaks the render.
Data type mismatch
The template expects a string but receives a number, a date, or an array. Some platforms coerce the type gracefully. Others render the raw type name or throw an error.
Why it matters
Broken merge tags are brand damage in the inbox
A welcome email that opens with "Hi" instead of "Hi Sarah" reads as impersonal but forgivable. An order confirmation that says "Your order null has shipped" or "Your order {{order.number}} has shipped" looks broken. The customer either ignores it or contacts support, and your team spends time troubleshooting something that should have been caught before send.
Merge tag breakages are often silent. The platform reports a successful send. The email arrives in the inbox. The dashboard shows green. The only signal something went wrong is a customer complaint or, more often, no signal at all because the customer just moves on.
Real example
A field rename that broke live flows
A broadcaster running a Saturday-morning newsletter merged a new CDP into their sending stack. The CDP renamed the subscriber timezone field from "tz" to "timezone_id". The template still referenced "tz". The platform sent the emails on schedule. Every greeting line rendered as "Good morning, {{subscriber.tz}}" for the first hour of the campaign window.
The inbox monitor caught it within two minutes. The team paused the flow, updated the template reference, and resumed. Total affected sends: under 400. Without receive-side checks, the entire Saturday batch would have gone out broken.
Detection
How Telltide catches broken merge tags
Telltide monitors live journeys with test identities seeded into each flow. When a monitored email arrives, Telltide parses the body and checks for unresolved tags, null placeholders, or syntax remnants. If found, an alert fires immediately. The check runs on every monitored send, so a broken merge tag is flagged on the first occurrence, not after customer complaints accumulate.
This approach works across any sending platform because the detection happens at the inbox, not in the ESP's logs. The monitor does not care whether your platform uses Liquid, Handlebars, or proprietary syntax. It sees what the customer sees.
Related reading
Concepts that travel with merge tags
- Silent send: when the merge tag breaks so badly the email does not arrive at all.
- Triggered flow: the surface where merge tag breakages most often occur.
- Heartbeat monitoring: cadence-based detection, less useful for merge tags than per-send arrival checks.
- Glossary home: full list of receive-side monitoring terms.
Further reading
Merge tag breakage in context
- RFC 5322: the Internet Message Format standard, which does not define merge tags but does define the message body where broken tags appear.
- Telltide home: platform-agnostic monitoring that catches merge tag breakages before customers do.
Catch broken merge tags before they reach customers
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