Definition
A custom field is a data field in your email platform that you define to store additional information about subscribers beyond the default fields (email, name). Custom fields store any subscriber-specific data that is relevant to your email programme — company name, job title, purchase date, product interest, or any other data point.
Custom fields are the foundation of email personalisation and segmentation. Without custom fields, you can only personalise by name and basic data.
Common Custom Field Uses
| Field Type | Example | Use Case |
|---|---|---|
| Contact info | Phone number, company, job title | B2B segmentation |
| Preference | Topic interest, frequency choice | Preference-based targeting |
| Behavioural | Last purchase date, total spent | Behavioural segmentation |
| Lifecycle | Subscription tier, signup date | Lifecycle campaigns |
| Demographic | Location, age, industry | Demographic targeting |
| Custom | Any data unique to your business | Specific needs |
Custom Field vs Merge Tag
| Concept | Purpose | Example |
|---|---|---|
| Custom field | Stores the data | last_purchase_date: 2026-07-15 |
| Merge tag | Displays the data in email | {{last_purchase_date}} |
You define custom fields in your ESP to collect and store data. You use merge tags in templates to display that data in emails.
Custom Field Best Practices
- Plan your fields before building: Define what data you need before you start collecting it. Avoid creating fields ad hoc.
- Use consistent naming: Use snake_case or camelCase consistently. Avoid spaces in field names.
- Choose the right field type: Use date fields for dates, number fields for numbers, text for text. Correct types enable better filtering.
- Set default values: Provide default values for fallback when data is missing. "Friend" as a name fallback prevents embarrassing blank greetings.
- Document your fields: Maintain a data dictionary that describes each field, its format, and where it comes from.
Custom Field Limitations
| Limitation | Impact | Workaround |
|---|---|---|
| Field count limit | ESPs cap the number of custom fields | Prioritise essential fields |
| Data type restrictions | Some fields require specific formats | Validate data before import |
| Sync complexity | Custom fields require integration setup | Use API or automated sync |
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Frequently Asked Questions
As many as you need, but fewer than you want. Each custom field adds complexity to data collection and maintenance. Start with 5-10 essential fields. Add more as your data needs grow.
A custom field stores a specific value (date, number, text). A tag is a simple label applied to a subscriber. Custom fields enable filtering by value ranges. Tags enable filtering by category membership.
Through signup forms (user provides data), CRM/API sync (data from other systems), behavioural tracking (ESP captures data), or import (CSV upload with field mapping).
Yes. Custom field values can trigger conditional content blocks in your email templates. "IF lastpurchasedate > 90 days SHOW re-engagement offer" is a common example.
When a subscriber is deleted, their custom field data is also removed. If you need to retain anonymised data, export it before deletion.