How to import and clean B2B lead lists in Mailrush for maximum deliverability

If you send cold emails, you know the nightmare: you buy or build a lead list, load it into an email tool, and… your open rates tank. Messages bounce. You get flagged as spam. If that sounds familiar, this guide is for you. Here’s how to import and clean B2B lead lists in Mailrush without nuking your sender reputation. I’ll walk you through what matters, what doesn’t, and where most people screw up.

Step 1: Don’t Just Dump Your List—Inspect It First

Let’s be blunt: most lead lists suck. Even if you bought it from a reputable source, expect bad data. Duplicates, typos, and dead domains love to sneak in.

Before you touch Mailrush, open your CSV and check:

  • Are the columns clear? You want “Email,” “First Name,” “Company,” etc. If it’s a mess, clean it up in Excel or Google Sheets.
  • Obvious junk: Any emails like test@test.com, info@, or support@? Nuke them. They almost never convert and can hurt deliverability.
  • Duplicates: Sort by email and remove any repeats. Mailrush can do some de-duping, but don’t rely on it.
  • Weird formatting: Watch for stray spaces, odd symbols, or emails pasted wrong. Fix them now—otherwise, you’ll pay for it later.

Pro tip: If you see a lot of free email providers (Gmail, Yahoo, Hotmail), be cautious. For B2B, stick to company domains. Personal emails are often spam traps or unqualified leads.

Step 2: Scrub Your List With a Real Email Verifier

If you skip this, you’re playing Russian roulette with your sender reputation. Mailrush has basic cleaning, but for real hygiene, use a dedicated email verifier before you upload.

What to Use

  • ZeroBounce, NeverBounce, BriteVerify—they all do the job.
  • Don’t get sucked into “AI-powered” list cleaning tools that focus on enrichment instead of validation. You want bounces gone, not just more data fields.

How to Do It

  1. Export your cleaned CSV.
  2. Upload it to your chosen verifier.
  3. Download the results—ideally, a list marking each email as “valid,” “invalid,” or “catch-all.”
  4. Delete: Remove all “invalid” and “unknown/catch-all” emails. If you’re desperate, you can keep “catch-all,” but know you’re risking more bounces.

Why bother? - High bounce rates (over 3-5%) will get your domain blacklisted. - Some dirty lists are full of spam traps. If you hit those, you’re toast.

Step 3: Prep Your Data for Mailrush Import

Mailrush is picky about its import format. If you ignore the template, you’ll waste time troubleshooting.

What Mailrush Wants

  • CSV file, UTF-8 encoding.
  • Clearly labeled columns—at minimum, “Email.” Add more columns if you want to personalize emails (name, company, etc.).
  • No blank rows.
  • No weird characters or line breaks in the data.

Common Screw-ups

  • Non-CSV files (Excel .xlsx won’t work)
  • Commas inside data fields (wrap those cells in quotes)
  • Missing headers

If you’re unsure, download Mailrush’s sample template and match your columns to it.

Step 4: Import Into Mailrush and Map Your Fields

Now, finally, into Mailrush.

  1. Go to the Lists section.
  2. Click “Import List.”
  3. Upload your cleaned CSV.
  4. Map your columns: This tells Mailrush which column is “Email,” “First Name,” etc. Don’t skip this—bad mapping means broken personalization.

Watch Out For

  • If you get a high failure rate on import, your file probably isn’t formatted right.
  • Mailrush will skip obvious duplicates and malformed emails, but don’t rely on it to catch everything.
  • If you see a big gap between “rows imported” and “rows uploaded,” re-check your CSV for junk.

Step 5: Clean Inside Mailrush (But Don’t Rely On It Alone)

Mailrush does offer built-in cleaning, but it’s basic. It’ll catch bad formatting, duplicate emails, and some obvious fakes. That’s not enough for serious cold outreach, but it’s a decent last filter.

  • After import, run Mailrush’s “Clean List” function.
  • Remove any emails flagged as invalid or risky.
  • Don’t ignore warnings—if Mailrush says an email looks bad, it probably is.

Honest take: This step is a backup, not a substitute for proper cleaning before import. Think of it as your safety net.

Step 6: Segment and Warm Up Your List

You’ve got a clean list—don’t blow it by blasting everyone at once.

Segment Smartly

  • Break up your list by company size, geography, or persona if you can.
  • Smaller sends = less chance of getting flagged as spam.
  • You can personalize your message better this way, which helps deliverability and reply rates.

Warm-Up Sends

  • Start with small batches (50-100 per day), especially if your sending domain is new.
  • Gradually ramp up over 1-2 weeks.
  • This signals to inbox providers that you’re not a spammer dumping a giant list.

Ignore the urge to “just get it over with” and send to everyone at once. You’ll just end up in spam or worse, get your account suspended.

Step 7: Monitor Your Results and Prune Aggressively

Even the best list gets stale. Some emails will bounce anyway. Some prospects will never open.

  • Watch bounce rates: Over 3-5%? Pause and re-clean.
  • Track opens and clicks: Remove anyone who never engages after a few sends. Dormant contacts are dead weight.
  • Hard bounces: Remove immediately. Soft bounces—monitor, but don’t panic unless they persist.

Don’t Obsess Over Vanity Metrics

Open rates are getting less reliable thanks to privacy changes (Apple Mail, Gmail, etc.). Focus on replies and actual conversations. That’s what matters.

What to Ignore (And What Not to Stress About)

There’s a lot of noise about “super-enriched” lead lists and AI-driven cleaning. Most of that is marketing fluff. Here’s what you can safely ignore:

  • Appending dozens of data fields: More data doesn’t mean better deliverability. Stick to what you’ll actually use.
  • “Secret” blacklist checkers: Most are unreliable. Focus on bounce rates and engagement instead.
  • Over-personalization: First name and company are usually enough. You don’t need to reference someone’s lunch order.

Summary: Keep It Clean, Keep It Simple

Importing and cleaning B2B leads in Mailrush isn’t rocket science, but skipping steps will burn you—fast. Focus on:

  • Starting with the cleanest list you can get,
  • Verifying with a real tool,
  • Keeping your imports tidy,
  • Sending in small, targeted batches.

No need to overthink it. Clean, send, learn, repeat. The less you try to game the system, the better your results will be. Stay skeptical, iterate, and don’t be afraid to prune hard. Your inbox placement (and your sanity) will thank you.