How to Clean a CSV for Mailchimp Import

1. Use Mailchimp's expected column names

Mailchimp recognises these column headers automatically: Email Address (required), First Name (FNAME), Last Name (LNAME), Address (MERGE3), Phone (PHONE). If your headers don't match, Mailchimp asks you to map them manually — which works, but errors are easy when you have 20+ columns.

Rename your CSV headers to match before importing. For custom MERGE fields you've added to your audience, use the MERGE tag name (MERGE5, MERGE6, etc.) or the field label exactly as configured in Audience → Settings → Audience fields.

2. Clean and deduplicate email addresses

Mailchimp uses the Email Address field as the unique identifier. A duplicate email in your import file won't create two contacts — Mailchimp merges them — but the last row wins, silently overwriting first name, tags and other fields from the earlier row.

• Trim spaces and lowercase every address (Mailchimp is case-insensitive but lowercase is cleaner).
• Remove clearly invalid addresses: missing @, missing domain, test@test.com entries.
• Remove duplicates within the file, keeping the most complete row.
• If you have a bounce list from a previous send, remove those addresses — importing known bounces hurts your sender reputation and Mailchimp may suspend your account for high bounce rates.

3. Remove unsubscribed and cleaned contacts

If your source list includes people who previously unsubscribed from your Mailchimp audience, importing them will not re-subscribe them — Mailchimp protects unsubscribe status. However, importing contacts that Mailchimp previously 'cleaned' (hard bounced) will add them back to your audience count without fixing the underlying deliverability issue.

Before importing, cross-reference against your Mailchimp export of cleaned and unsubscribed contacts. Remove any matches from the import file.

4. Format tags and groups

Mailchimp supports Tags and Groups (interest categories). During import, you can assign tags to all imported contacts, but you can also include a Tags column in your CSV. Multiple tags in one cell should be separated by commas.

• Standardize tag names: 'webinar-attendee' and 'Webinar Attendee' would create two separate tags.
• Group assignments use columns named after the group category, with values matching group names exactly.
• If you're migrating from another platform, map status fields (lead, customer, VIP) to Mailchimp tags rather than trying to use custom MERGE fields for segmentation.

5. Check MERGE field data types

Each MERGE field in Mailchimp has a type: text, number, date, birthday (MM/DD), zip code, URL, phone, address. Values that don't match the type will fail silently — the field will be blank for that contact.

• Date fields must be in MM/DD/YYYY format (US date order, regardless of your locale).
• Birthday fields use MM/DD only.
• Phone numbers are stored as text — any format works, but E.164 is best for SMS integrations.
• Address fields expect a structured format: Street, City, State, Zip, Country on one line separated by two spaces or in separate columns.
• Number fields reject values with currency symbols or thousand separators.

Pre-import checklist

Run your list through TheSpreadFix to catch duplicates, invalid emails and inconsistent formatting in one scan.