Create accurate, production-ready email verification skills for Claude Code, OpenCode, Gemini CLI, and custom AI agents. Learn why SMTP verification is the only reliable method and how to implement it properly.
Why most approaches fail and what actually works
When building AI agent skills for email verification, developers face a fundamental challenge: most verification approaches cannot determine if an email actually exists.
Regex patterns can validate email format but cannot tell you if the mailbox exists. An email like valid-format@nonexistent-domain.xyz passes syntax checks but is completely invalid.
Checking if a domain has MX records confirms the domain can receive mail, but says nothing about whether a specific mailbox exists. user@gmail.com and doesnotexist@gmail.com both have valid MX records.
Many email verification APIs only perform syntax and DNS checks, then use statistical models to guess validity. This leads to 20-40% false positives in real-world scenarios.
What makes real email verification possible
SMTP verification directly connects to the mail server and checks if the mailbox exists. This is the only way to truly confirm email deliverability.
Connect to the recipient's mail server on port 25
Establish an SMTP handshake
Send the RCPT TO command with the email address
Receive confirmation (250) or rejection (550) from the server
Close connection without sending actual mail
AWS, GCP, Azure, DigitalOcean, and most cloud providers block outbound port 25 by default. Even if you request an unblock, you'll be heavily rate-limited and quickly blacklisted.
Mail servers reject connections from unknown IPs. Building IP reputation takes months of legitimate sending. Without it, your verification requests are ignored or blocked.
Modern mail servers detect verification attempts. Without proper infrastructure and relationships, you'll hit rate limits, CAPTCHAs, and outright bans.
Reliable SMTP verification requires multiple IPs across different subnets, automated IP rotation, retry logic, timeout handling, and 24/7 monitoring. This is a full infrastructure challenge.
Complete guide for different AI frameworks
Create a custom skill that leverages EmailVerify's MCP server for accurate email verification.
Add email verification capabilities to OpenCode workflows with a single configuration.
Enable Gemini CLI to verify emails using the MCP protocol.
Build LangChain agents with email verification tools for automated workflows.
Create data quality agents with built-in email verification.
Integrate with any agent framework using our REST API or MCP server.
Build reliable email verification capabilities
Set check_smtp: true in your verification calls. Without SMTP verification, you're only getting syntax and DNS checks, which miss 20-40% of invalid emails.
Process valid, invalid, and unknown status results appropriately. Unknown results may need retry with SMTP enabled.
Use verify_batch_emails for multiple emails instead of individual calls. This is 5-10x more efficient.
Don't repeatedly verify the same email. Our 7-day cache for valid/invalid results prevents waste and speeds up responses.
Use get_account_balance before large batch operations to ensure sufficient credits.
Enterprise infrastructure built for AI agent use cases
We operate dedicated infrastructure with whitelisted IPs across multiple regions. Port 25 access with excellent sender reputation.
First email verification service with MCP server. One-line configuration for Claude Code, OpenCode, Gemini CLI.
Real mailbox verification, not statistical guessing. Know for certain if an email exists.
Only pay for valid emails. Invalid and unknown results are free. No wasted credits.
Under 2 seconds for single emails. Smart caching for instant repeated lookups.
Servers worldwide for optimal verification of any email domain.
| EmailVerify | DIY | |
|---|---|---|
| SMTP Verification | Full SMTP verification with whitelisted IPs | Blocked by cloud providers, no IP reputation |
| Accuracy | 99.9% - real mailbox confirmation | 60-80% - syntax/DNS only |
| Setup Time | 5 minutes - one configuration line | Weeks to months - infrastructure + reputation building |
| Maintenance | Zero - fully managed service | Ongoing - IP rotation, blacklist monitoring, retry logic |
| Cost | Pay per verification - no infrastructure cost | Dedicated servers + multiple IPs + monitoring + time |
Add our MCP server: claude mcp add emailverify -- curl --stdio "https://mcp.emailverify.ai/mcp?api_key=YOUR_KEY". Then ask Claude to verify any email. No SDK installation needed.
Yes. Add MCP config to ~/.opencode/config.json. OpenCode fully supports MCP servers for email verification with real SMTP checking.
Go to Settings > MCP in Cursor or Windsurf, add the emailverify config. Both editors natively support MCP for AI agent skills.
Syntax and DNS checks miss 20-40% of invalid emails. Only SMTP verification confirms mailbox existence with 99.9% accuracy. Cloud providers block port 25, so you need specialized infrastructure.
AWS, GCP, Azure block port 25 by default. Even with unblocking, you lack IP reputation and will be blacklisted. Building reliable SMTP infrastructure takes months of IP warming.
All major frameworks: Claude Agent SDK, LangChain, CrewAI, AutoGPT. MCP works with Claude Desktop, ChatGPT, DeepSeek, Kimi, plus coding tools like Cursor, Windsurf, Cline.
Agent skills are reusable capabilities AI agents invoke. EmailVerify gives Claude Code, OpenCode, Gemini CLI the ability to verify emails via natural language commands.
Pay only for valid emails. Invalid and unknown results cost 0 credits. Start free with 100 credits, no credit card required.
Join developers using EmailVerify to build production-ready agent skills with real SMTP verification.