SMTP Limits, Sending Rates & Optimization

Introduction

SMTP (Simple Mail Transfer Protocol) is the backbone of email communication, but improper use can lead to blocked emails, poor deliverability, and sender reputation damage. mySMTP enforces sending limits, rate controls, and warm-up procedures to protect email servers and ensure messages reach inboxes.

This entry covers:
✅ SMTP limits & sending rates
✅ IP warming strategies
✅ Rate limiting by email provider
✅ MailWizz quota settings
✅ SMTP error codes & troubleshooting
✅ Best practices for high-volume email sending

1. SMTP Sending Limits & Rate Control

Understanding SMTP Limits

SMTP providers enforce limits to prevent abuse. These limits vary based on:

• Subscription Plan (Basic vs. Enterprise)
• IP Type (Dedicated IPs have higher limits than shared IPs)
• Email Volume (Sudden spikes can trigger throttling)
• Recipient Domain (Different providers enforce different limits)
• Bounce & Spam Rate (High bounces lead to automatic slowdowns)

💡 Exceeding limits may result in:

• Temporary throttling (slowing down emails).
• Deferrals (email sent later, not immediately).
• Blocking (emails rejected if too many complaints/bounces).

2. IP Warming: Increasing Limits Gradually

Why Warm Up your IP?

New IPs have no reputation and are monitored closely by email providers (Gmail, Outlook, Yahoo, etc.). Sending too many emails at once from a fresh IP looks suspicious and may lead to temporary blocking or blacklisting.

IP Warming Plan (Example: 100,000 Emails/Day Goal)

📌 Best Practices:
✔ Start with engaged recipients (those who open emails often).
✔ Avoid purchased or scraped email lists (can trigger spam complaints).
✔ Maintain a steady sending schedule (no sudden spikes).
✔ Authenticate emails with SPF, DKIM, and DMARC.

3. SMTP Rate Limiting by Destination

Different email providers enforce their own sending limits, affecting how fast mySMTP can relay messages.

🚀 How to Avoid Rate Limits:
✅ Separate email streams (transactional vs. marketing).
✅ Use subdomains (e.g., news.example.com for campaigns).
✅ Monitor feedback loops (check complaints & bounces).

4. MailWizz Minute Quota Configuration (For Bulk Senders)

MailWizz allows you to control SMTP sending speed to avoid exceeding limits. (in progress)

5. SMTP Error Codes & Troubleshooting

When hitting SMTP limits, you may see these errors:

📌 Pro Tip: Enable SMTP logs in your email software to track failures.

6. Advanced Deliverability Tips

🔹 Improve Reputation & Deliverability:
✔ Use dedicated IPs for high-volume sending.
✔ Segment by recipient domain (separate Gmail, Yahoo, etc.).
✔ Monitor complaint rates (keep spam complaints < 0.3%).
✔ Implement engagement-based sending (send more to users who open emails).

🔹 Avoid Getting Blocked by Gmail & Outlook:
✔ Use SPF, DKIM, and DMARC authentication.
✔ Don’t use spammy words in subject lines (free, win, urgent).
✔ Keep unsubscribe links visible to prevent spam reports.
✔ Monitor Postmaster Tools (Gmail) & SNDS (Microsoft) for complaints.

7. Scaling Beyond Limits (Enterprise Solutions)

If you need to send 1M+ emails/day, consider:

• Multiple mySMTP IPs (rotate between different servers).
• Dedicated sending domains (separate marketing & transactional emails).
• AI-driven sending strategies (adjust rates based on engagement).
• mySMTP postmulti servers (spreading email traffic across multiple IPs).

💡 Enterprise users can request custom rate limits and priority SMTP queues via mySMTP support.

Conclusion

Understanding SMTP limits, warming up IPs, and managing sending rates is critical to avoiding blocks and ensuring high deliverability.

🔹 Need custom SMTP settings or higher limits? Contact mySMTP support for tailored recommendations. 🚀