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OA-4 · Outbound Automation · 100 XP · ~18 min
A 5% bounce rate sounds small. But it compounds: mailbox providers flag you as a low-quality sender, your deliverability drops, your open rates fall, and eventually your emails stop reaching inboxes entirely. Email validation is the unglamorous work that keeps the machine running. This module covers the four validation layers you need, how to build them in Bitscale, and what to do with the results.

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The Four Validation Layers

Not all validation is equal. Most teams only do one or two layers and wonder why their bounce rates creep up.
LayerWhat It ChecksWhen to Run
SyntaxValid email format (has @, valid domain structure)Always, first
DomainDoes the domain exist and accept email? (MX record check)Always
Catch-all detectionDoes the server accept all emails regardless of whether the mailbox exists?Before enrichment
Mailbox verificationDoes this specific mailbox exist? (SMTP probe)Final layer, before sending
Running all four layers before sending is table stakes for any serious outbound program.

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Catch-All Domains: The Hidden Problem

A “catch-all” domain is configured to accept email addressed to any address at that domain — even if the mailbox doesn’t exist. This means validation services will return “valid” for sdflkjsdf@catch-all-company.com even though nobody checks that inbox. Catch-all addresses are common at mid-market and enterprise companies. If you’re seeing high bounce rates despite “validated” lists, catch-all addresses are usually the culprit. How to handle catch-all addresses in Bitscale:
  1. Tag them separately in a validation_status column
  2. Apply a separate sending strategy: lower volume, more conservative sending windows
  3. Or deprioritize entirely if you have enough valid addresses in the segment

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Building a Validation Column in Bitscale

The cleanest approach is a dedicated validation column that runs an external API, then a routing column that sorts addresses into buckets. Step 1: Add an API integration column Connect to your preferred validation provider (NeverBounce, ZeroBounce, Millionverifier). The Bitscale enrichment column can call any API that accepts an email address and returns a status. Input: {{email}} Step 2: Map the response to a status bucket Your AI column processes the API response into clean categories: 
Based on this email validation API response: {{api_response}}

Classify this email as one of:
- valid: mailbox confirmed to exist
- invalid: mailbox does not exist
- catch-all: domain accepts all mail
- risky: cannot be confirmed (timeout, firewall, etc.)
- undeliverable: hard bounce

Return ONLY the classification word.
Step 3: Routing logic Add a column for send/hold/skip logic: 
Given validation_status = {{validation_status}}, classify as:
- send: if valid
- send_carefully: if catch-all (limit to 20/day from this domain)
- skip: if invalid or undeliverable
- manual_review: if risky

Return ONLY the routing decision.

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Validation at the Domain Level

Before you even verify individual mailboxes, you can filter at the domain level. This is much cheaper (no per-address API cost) and removes obvious dead weight. Domains to filter immediately:
  • Free email providers: gmail.com, yahoo.com, hotmail.com, outlook.com (for B2B)
  • Role-based addresses: info@, hello@, admin@, support@, sales@ (usually unmonitored)
  • Known spam traps: check against blacklists
  • Domains with no MX records: the company literally doesn’t have email set up
Bitscale column for domain-level filtering: 
Email: {{email}}

Extract the domain from this email. Then classify:
- personal_email: if it's a common free provider (gmail, yahoo, hotmail, outlook, icloud)
- role_based: if it starts with info, hello, admin, support, sales, contact, noreply, no-reply
- business: everything else

Return ONLY: personal_email, role_based, or business
Run this column first. It costs nothing (no API call) and typically removes 10–20% of a raw list before you spend a penny on validation.

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Bounce Rate Management

Even with perfect validation, you’ll get some bounces. The goal is staying under thresholds:
MetricGreenYellowRed — Stop sending
Hard bounce rate< 1%1–2%> 3%
Soft bounce rate< 3%3–5%> 8%
Spam complaint rate< 0.08%0.08–0.3%> 0.3%
If you hit red on any metric, pause sending immediately, investigate the source of bounces, re-validate the affected segment, and restart at reduced volume.
Google’s 2024 sender requirements set a 0.3% spam complaint rate as a hard threshold for bulk senders. Exceed it and your emails stop reaching Gmail inboxes. Monitor complaint rates in Google Postmaster Tools — not just in your sequencer.

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Validation Workflow: The Full Sequence

Here’s the complete validation pipeline as a Bitscale grid flow:
  1. Import raw list → your source data
  2. Domain filter column → remove personal/role-based emails
  3. MX record check column → remove domains with no mail server
  4. Catch-all detection column → tag catch-all domains
  5. Mailbox verification column → SMTP probe on remaining addresses
  6. Routing column → send / send_carefully / skip / manual_review
  7. Export filtered list → only send and send_carefully rows go to sequencer
This pipeline typically removes 15–35% of a raw list. That’s not wasted effort — it’s saved sender reputation.
Quick Check: What’s the difference between a syntax check and a mailbox verification? What is a catch-all domain and why is it a problem? What bounce rate triggers immediate sending pause?

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OA-4 Challenge: Build a Validation Pipeline (+100 XP)

Take a list of at least 50 email addresses (use a sample from a public dataset or generate test addresses) and build the full 4-layer validation pipeline in Bitscale. Requirements:
  • At least 4 validation columns (syntax, domain filter, catch-all detection, routing)
  • A validation_status column with clean bucketed output
  • A routing_decision column (send / send_carefully / skip / manual_review)
  • Screenshot showing the distribution of each bucket
  • Brief paragraph explaining your decision-making on the catch-all addresses

Submit OA-4 Challenge →

Share your grid link + screenshot of bucket distribution. +100 XP on approval.

Next: OA-5 — Multichannel Orchestration →

Email is one channel. OA-5 covers how to coordinate LinkedIn, email, and phone into a single orchestrated outbound motion.