Prepare a marketing contact list without uploading it

A newsletter export here, a webinar list there, and a separate file of people who opted out. Preparing those files should be a transparent process—not a black box that promises a “perfect” audience.

Our free browser tools handle the file preparation. Your email platform still handles permissions, current unsubscribe records and sending. Start with copies of your exports and keep the source files unchanged.

1. Bring the exports together

Use the Marketing CSV Merger to combine files. Review the output column names first: “Email” and “Email address” can describe the same field, while “Signup date” and “Last purchase date” should stay separate.

Preserve names, company details, list source and subscription fields. Missing values remain blank. The optional duplicate setting removes only completely identical rows; it does not decide which of two conflicting contact records is newer or more reliable.

2. Review duplicates and formatting

The Marketing Email List Cleaner checks basic address formatting and optionally removes repeated addresses. It trims outer whitespace and lowercases the domain while leaving plus tags and dots intact.

Always inspect the review report. An unusual address format is not proof of a nonexistent inbox. The cleaner does not test mailbox existence or predict inbox placement.

Duplicate handling keeps the first row. If one row says “Subscribed” and another says “Unsubscribed,” resolve that conflict in your source system before preparing a campaign.

3. Apply your exclusions

Export the current unsubscribe, bounce or do-not-contact list from your campaign platform. In the Suppression List Checker, select the campaign email column and the exclusion email column.

Every address in the exclusion file is treated as blocked. Matching ignores letter case by default, but does not remove dots or plus tags. If an exclusion entry has unsupported formatting, the export stops for review rather than silently ignoring it.

The remaining rows are simply addresses that did not match this file. They are not automatically subscribed, verified or eligible for outreach. The tool does not interpret status columns. Recheck current permissions and suppressions in your platform before sending.

4. Split for import

If your next system accepts a limited number of rows per import, use the Marketing CSV Splitter. Choose the number of data rows per part; each part repeats the header when selected.

The ZIP includes a report of each part and its row count. The tool preserves order and does not clean or remove records. Start with one small test import and check that the destination maps the columns correctly.

What stays local—and what to keep in mind

• Selected file contents are processed in browser memory, without file-content uploads or a mailbox connection.
• Clear the tool or leave its page to release its working state. Saved downloads remain on your device.
• CSV exports escape spreadsheet formula prefixes. Those affected cells may have a leading apostrophe; check your destination’s import behavior.
• Each tool shows its file and row limits. These tools handle CSV/TXT/TSV exports, not Excel workbooks.
• Local processing is a privacy feature, not a guarantee of marketing compliance. It does not replace your organization’s permission checks.

Explore the free tools or start with a built-in example. No purchase or signup is required.