For lead gen agencies managing thousands of contacts across multiple accounts, bad email addresses are a constant threat that can damage the quality of your deliverables and waste your clients’ budgets.

The easiest way to protect your infrastructure and ensure clean lists at scale is with an email verification tool.

This article covers six email verifiers built for agencies handling massive lists.

We’ve highlighted the features that matter most for high-volume operations and broken down what makes each tool worth considering for your agency’s workflow.

1.  Instantly

Instantly is an email verification and outreach platform used by lead gen agencies to clean large contact lists and generate qualified leads for clients. It verifies emails in bulk or individually, tagging each lead as valid, risky, or invalid so your team knows exactly which contacts to pursue.

You can upload large CSV files or verify leads already loaded into the platform. The platform also includes catch-all verification, which checks hard-to-validate email addresses that traditional verifiers often miss. You only pay for validated leads, and verification credits never expire, so there’s no rush to use them before they reset.

For agencies that need verified contacts ready to go, Instantly includes Supersearch, a database of 450 million verified business contacts that plugs directly into its verification system. Every contact exported from Supersearch is automatically validated before it reaches your campaign, so there’s no need for separate tools or manual uploads.

Alongside its verification features, the platform also offers email warming across a network of over 1 million real accounts, campaign automation, and built-in CRM features. For agencies that also handle outreach as part of their lead generation process, it’s a one-stop solution where they can verify, warm, send, and qualify leads instead of managing separate subscriptions for each step.

Feature Highlights

• Verify single contacts or entire lists through CSV upload
• Catch-all detection handles addresses that most tools can’t validate
• Credits stay in your account forever with no expiration
• Labels every email as valid, risky, or invalid for quick filtering
• Works natively with Supersearch database and campaign builder

Pricing

• Starts at $17/month for 5,000 verification credits
• Supersearch database access starts at $47/month
• Cold email outreach plans start at $37/month

2.  MailerCheck

MailerCheck verifies emails by letting you upload a spreadsheet, connect your ESP, or use the API for real-time checks. Single email verification is also available, and the verification process runs quickly with results populating as the check progresses.

Instead of simple pass/fail labels, MailerCheck breaks down email statuses into allowed, risky, and do not send categories, then shows the exact reason behind each classification. Once verification is completed, exporting doesn’t require manual cleanup. Select the categories you want to include and either push addresses back to your connected tool or download a file ready for upload. The platform skips duplicate emails automatically, so you’re not charged twice if the same address appears multiple times across a massive list.

MailerCheck also includes deliverability tools like inbox placement testing, spam content analysis, blocklist monitoring, and DMARC tracking to help agencies catch issues before they hurt sender reputation.

If you use MailerLite for email marketing, MailerCheck offers single sign-on, so you can access both platforms with the same login.

Feature Highlights

• Granular status reporting with exact reasons for each email classification
• Bulk export options that filter by category
• Automatic duplicate detection across large lists
• Inbox placement predictions and spam content analysis

Pricing

• Starts at $10 for 1,000 credits
• Cost per credit ranges from $0.01 to $0.001, depending on volume

3.  Kickbox

Kickbox categorizes emails as deliverable, risky, undeliverable, or unknown, and breaks them down further into role-based, disposable, and catch-all addresses. The interface is simple and reports are easy to read, which matters when you’re processing lists with tens of thousands of contacts.

The Sendex score helps agencies make decisions about borderline contacts at scale. This algorithm assigns a quality score to each email address, so you can set thresholds and filter out low-scoring leads in bulk rather than reviewing them one by one. You can also compare average Sendex scores across different lists to spot which lead sources are delivering better quality before you scale up purchases.

Kickbox integrates with popular ESPs and offers API access for real-time verification. The platform is also GDPR compliant and guarantees 95% deliverability on verified emails.

Feature Highlights

• Sendex quality scoring for individual emails and full lists
• Detailed categorization, including role-based and catch-all flags
• API and ESP integrations for automated workflows
• 95% deliverability guarantee

Pricing

• Starts at $5 for 500 emails
• Cost per email ranges from $0.01 to $0.004

4.  MillionVerifier

MillionVerifier covers the essentials: single and bulk verification, API access, and detailed reports. It integrates with 17 major ESPs, so there’s a decent chance your platform is already supported.

Reports split results into good, risky, and bad categories, and you can download whichever segments you need. The automation feature is worth noting for agencies managing multiple client accounts. You can set Million Verifier to check subscriber lists on connected email platforms automatically at regular intervals, which saves time when you’re overseeing dozens of campaigns that need consistent list hygiene.

The platform is GDPR compliant and guarantees 95% deliverability on verified emails. If you connect your ESP, you get free verification credits for up to 5% of your total contact list, which can mean thousands of free verifications for agencies managing large databases.

Feature Highlights

• Automated verification schedules for connected ESPs
• Good, Risky, and Bad segmentation with downloadable reports
• Integration with 17 email platforms
• Free credits for up to 5% of contacts when ESP is connected

Pricing

• Starts at $4.90 for 2,000 emails
• Cost per email starts from $0.00245

5.  ZeroBounce

ZeroBounce verifies email lists through direct integrations with major ESPs or CSV uploads. Reports group email addresses by issue type and are easy to scan when you’re reviewing large batches.

The platform also pulls the user’s name and guesses their gender directly from the email address. For agencies running cold outreach across multiple clients, this saves time on personalization and merge tag setup, especially when you’re working with lists in the tens of thousands.

The platform includes deliverability features beyond verification and is GDPR compliant. It also guarantees 99% deliverability on verified emails, the highest on this list.

Feature Highlights

• Name and gender detection from email addresses for personalization
• Easy-to-read reports grouped by issue type
• Integration with popular ESPs and API access
• 99% deliverability guarantee

Pricing

• Starts at $18 for 2,000 emails
• Cost per email ranges from $0.009 to $0.00225, depending on volume

6.  NeverBounce

NeverBounce includes a list analysis feature that estimates list health before you run a full verification. This free scan tells you whether cleaning would make a meaningful difference, which helps agencies avoid wasting credits on lists that are already in good shape.

The results page groups emails into valid, accept all, unknown, disposable, and invalid categories. You won’t see detailed explanations for why each email landed in a specific category, but you can choose which types to include or exclude during export. For agencies processing multiple client lists daily, this quick filtering keeps workflows moving without manual review. NeverBounce also integrates with popular ESPs, offers API access, and is GDPR compliant.

Feature Highlights

• Free list analysis to estimate health before verification
• Simple categorization with export filtering options
• Integration with popular ESPs and API access
• 97% deliverability guarantee

Pricing

• Starts at $8 for 1,000 emails
• Cost per email ranges from $0.008 to $0.003

Final Thoughts

The best way to decide is to test a few tools with their free credits. Run a small batch from one of your client lists and compare the results. Pay attention to how the tools categorize risky emails and whether the reporting makes sense for how your team works.

Another thing to consider is how the tool fits into your broader workflow. Some platforms only verify emails, while others (like Instantly) handle verification alongside lead sourcing and list management features. This can significantly streamline operations when you’re aiming to deliver clean, verified lists to multiple clients.

Start with Instantly’s free trial today to see how it handles your agency’s workflow.