You inherited a mailing list. Start here.

Before uploading an inherited list to a verifier, find the unsubscribe records. A fresh export can look reassuringly clean while quietly including people who asked you to stop emailing them.

Keep an untouched copy of the export. Then work through it in this order.

Find out where it came from

Look for signup dates, collection sources and records of what people agreed to receive. A missing source is a reason to investigate. It doesn't prove the list was bought, but it doesn't give you permission to use it, either.

Set aside contacts whose history you cannot establish. Verification won't fill that gap.

Apply exclusions before checking addresses

Exclude unsubscribers, spam complainants and addresses already suppressed for delivery failures. Keep those exclusions in your sending system so a later import cannot reactivate them.

Remove duplicate entries carefully. Preserve the most restrictive subscription preference when records disagree. Don't strip dots or plus tags from every address; different providers handle them differently.

Verify the remaining addresses

Read the reason behind each result before acting. A confirmed nonexistent mailbox belongs on your suppression list. A temporary failure may need a later check. A catch-all result needs context from your own records.

Keep the check date too. A result from last year tells you what was found last year.

Update the original system

Cleaning an export alone is temporary. Record the result in the CRM or sending platform, then check that your next export respects it. Suppressing an address is usually more useful than deleting its entire contact history.

For a dormant list, start with recent subscribers who still have a reason to expect your mail. Watch delivery failures and complaints before expanding the send. Stop and investigate if they rise.

You don't need to recover every row. You need a list you can explain and use responsibly.