What Are Custom Fields? Complete Guide | K2Mailer
Learn what custom fields are, how to create them, and use them to personalize emails and organize contacts in K2Mailer.
Quick answer: what are custom fields in email marketing?
A custom field is an extra piece of contact data you define yourself, such as company, plan type, city, or renewal date. In K2Mailer, a custom field has a name and a type, and it can be attached to one or more lists.
Once a field holds values, you can use it three ways: personalize emails with its shortcode, build segments that filter by its value, and collect it through web forms and imports. In short, custom fields turn a plain list of email addresses into data you can actually act on.
- Personalize: insert the value into subjects, content, sender values, or custom headers.
- Segment: filter list-based segments by field value or date.
- Collect: map import columns and add fields to subscription web forms.
- Organize: keep one consistent field for the same piece of data across lists.
Why custom fields matter
Generic email gets generic results. When a message uses details the subscriber actually gave you, it reads like it was written for them. That is the idea behind classic mail merge, and it is the foundation of every personalized campaign.
Custom fields also make audiences smarter. Instead of mailing everyone, you can reach customers on a particular plan, subscribers in one city, or contacts with a renewal date coming up. Smaller, more relevant sends tend to bring better engagement, and healthy engagement is good for deliverability. For more on that link, read the email marketing best practices guide.
There is a practical side too. When profile data lives in structured fields instead of a spreadsheet tab or someone’s memory, any teammate can find it, filter by it, and reuse it. New campaigns start faster because the audience building blocks already exist, and reports stay comparable from one send to the next.
How custom fields work
A custom field defines a named piece of contact data and can be attached to one or more lists. Field types support simple text as well as controlled values and dates.
Required fields must contain an acceptable value when a contact is saved through a workflow that enforces the field.
Custom field types and when to use them
- Text
- Free values such as company, job title, or city. Flexible, but typos creep in, so avoid it for anything you plan to filter on exactly.
- Dropdown
- A fixed list of choices, such as plan tier or industry. Good for clean segmentation because values stay consistent.
- Radio
- A single choice from a few visible options, such as a communication preference. Best for short lists on a web form.
- Date
- A calendar value such as a renewal, trial end, or birthday. Supports date-based segments and timely reminders.
Choose controlled types for anything you filter on
If you will segment by a value, a dropdown or radio field keeps the spelling consistent. “Pro” and “pro plan” as free text become two different audiences.
How to create and use a custom field
- 1
Decide what the field is for: personalization, segmentation, or reporting. If you cannot name its use, do not create it yet.
- 2
Open Contacts → Custom Fields and create the field with a clear, consistent name.
- 3
Choose the type: text, a controlled type such as dropdown or radio, or a date.
- 4
Attach the field to every list that should store this data.
- 5
Collect values by mapping a column during an import or by adding the field to a subscription web form.
- 6
Use the field in a segment or insert its shortcode into your campaign, with a fallback for empty values.
- 7
Preview with a contact that has a value and one that does not, then run the pre-send check before scheduling.
Where custom fields are used
- Map imported file columns to custom fields.
- Add the selected fields to subscription web forms.
- Filter list-based segments by field value or date.
- Insert the corresponding shortcode into subjects, content, sender values, or custom headers.
Personalize emails with custom field shortcodes
Every custom field has a matching shortcode. Insert it where you want the value to appear, and K2Mailer fills it in for each recipient at send time. A subject line that uses a city or a plan name usually stands out in a busy inbox.
Always set a fallback value. When a contact has no value for the field, the fallback supplies safe replacement text so the email never shows a blank or a raw placeholder. Choose a fallback that fits the sentence around it, because “Hi there” and “Dear Friend” need different words. The full workflow is in personalize campaigns with shortcodes.
Segment your audience with custom fields
List-based segments can filter by custom-field values and supported custom date fields. That lets you build audiences like “Pro plan customers in Lahore” or “renewal date in the next 30 days” and save them for reuse. A saved segment can be targeted directly from a campaign schedule, so you do not copy results into a temporary list before every send.
A date field can also drive timing. The same renewal date that powers a segment can feed a reminder campaign through triggers. Learn more in the segments guide and the email marketing automation article.
Planning a custom field strategy
Before creating a long list of custom fields, decide what each one will actually be used for: personalization in subject lines and body copy, segmentation logic for targeted sends, or reporting. Fields that exist “just in case” tend to stay empty, which weakens both personalization and any segment built on top of them.
Keep naming consistent across lists so the same piece of contact data, such as a renewal date or plan tier, always lives in a field with the same name and type. This matters most once you start importing files from multiple sources, since mismatched field names lead to data landing in the wrong place or not mapping at all.
Required fields are worth using sparingly, since they can block a web form submission or import row if the value is missing or improperly formatted. Review your custom fields periodically and retire ones that are no longer referenced by any segment, form, or campaign, so the contact editing screen stays easy to work with.
Custom field best practices
- Collect only what you will use. Every extra form field lowers sign-up completion.
- Prefer dropdown or radio fields for values you will filter on.
- Use one date format and never store a date as free text.
- Name fields the same way across lists, for example always “Plan,” never “Tier” in one list and “Plan” in another.
- Respect privacy. Do not store sensitive data you do not need, and keep in mind rules such as the GDPR on what you collect and why.
- Test every personalized email with an empty-value contact before sending.
Custom field examples by use case
The best custom fields mirror how your business actually talks to customers. A few starting points show how different teams put the same feature to work.
- SaaS and subscriptions
- Plan tier (dropdown), trial end date (date), and company (text). Segment trial users near expiry and personalize upgrade messages by plan.
- Online stores
- Favorite category (dropdown) and last order date (date). Send category-specific offers and win-back emails to lapsed buyers.
- Newsletters and publishers
- Topic interest (radio or dropdown) and source (text). Send the right edition and learn which channels bring engaged readers.
- Events and webinars
- Event name (text) and event date (date). Send reminders before the date and follow-ups after it.
- Agencies and B2B
- Industry (dropdown), job title (text), and account owner (text). Tailor outreach and route replies to the right person.
Frequently asked questions
- What are custom fields in email marketing?
- Custom fields are extra contact data points you define, such as company, city, plan, or renewal date. They let you personalize emails, build targeted segments, and keep subscriber data organized.
- Can one custom field be used across multiple lists?
- Yes, a custom field can be attached to more than one list and referenced consistently across them.
- What happens if a required field is left blank on import?
- A workflow that enforces the field will reject the row until an acceptable value is provided.
- Can custom fields drive both segments and personalization?
- Yes, the same field can filter a segment and also resolve as a shortcode inside campaign content.
- What if a contact has no value for a field I use in an email?
- Set a fallback value in the shortcode so the email shows safe replacement text instead of a blank. The pre-send check also catches malformed merge tags.
- Which custom field type should I choose?
- Use text for free values, dropdown or radio for fixed choices you will filter on, and a date type for renewals, trials, or birthdays so date-based segments work.
- Can I collect custom field data from a sign-up form?
- Yes. Add the selected fields to a subscription web form and subscribers fill them in when they join. See create a subscription web form.
- How many custom fields should I create?
- As few as you will genuinely use. Create a field only when you can name how it will personalize, segment, or report, and retire fields that no longer feed any segment, form, or campaign.
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