For iPost users sending newsletters, promotions, and bulk campaigns

Delivering emails to large audiences can be challenging. Even with permission-based lists, inbox providers like Microsoft, Yahoo, and AOL may apply filters, temporary blocks, or throttling. This guide helps iPost users understand common causes of delivery issues, SMTP error codes, and best practices to maximize inbox placement.


1. Why Deliverability Issues Happen

Inbox providers evaluate multiple factors:

  • Sender reputation (IP/domain)

  • Authentication alignment: SPF, DKIM, DMARC

  • Message content: risky keywords, formatting, spam-like language

  • Engagement metrics: opens, clicks, complaints, deletes

  • Volume and frequency: sudden spikes or large campaigns

Even well-intentioned senders can be filtered if these signals suggest spam or abuse.


2. Microsoft Limits Relevant to iPost Users

When sending to Outlook.com, Hotmail, Live.com, or MSN recipients, Microsoft enforces limits on messages from external senders (like iPost), but these limits are reputation-based rather than tenant-specific:

Limit Type
Details
Notes for iPost Users
Recipient limit per message
~500 recipients per single message
Segment large campaigns if sending to more than 500 Outlook.com/Hotmail addresses in one message
Daily recipient monitoring
Microsoft may throttle if a sender sends to large volumes of external recipients from a single IP/domain
Use iPost’s throttling options and segment campaigns to reduce risk
Connection limits / throttling
The number of messages accepted per minute/hour from a single IP may be limited
iPost sending infrastructure manages these automatically
Spam & content filtering
Emails with risky keywords, poor engagement, or failed authentication may be blocked or routed to spam
Ensure SPF/DKIM/DMARC are set, avoid gambling/money-heavy language, and maintain high engagement

These limits are primarily designed to protect recipient inboxes and maintain Microsoft’s email ecosystem reputation.


3. Yahoo & AOL Filtering and Best Practices

Yahoo and AOL share backend filtering infrastructure. They are strict with high-volume senders and gambling-related content.

Words and Phrases to Avoid

  • Gambling: bet, wager, casino online, jackpot, bonus, spin & win

  • Money: cash, win money, claim prize, double your money

  • Urgent/pushy: act now, last chance, click here, claim today

Safe Subject Line Example


✅ Good: {firstname}, Your November Offers Are Here
⚠️ Risky: Win BIG at our slot tournament this weekend!


Deliverability Tips for Yahoo/AOL

  • Focus on experiences and entertainment instead of cash prizes

  • Maintain clean lists; remove long-term inactive users

  • Keep a healthy text-to-image ratio (at least 30% text)

  • Test campaigns with seed accounts to check inbox placement


4. Common SMTP Error Codes

Code
Meaning
Typical Cause
Recommended Action
5.2.2
Mailbox full
Recipient inbox is full
Retry once; suppress if persistent
5.0.0
Permanent failure
Invalid address, typo, or policy block
Verify the email address and bounce text
5.5.4
Protocol or authentication issue
Misalignment with recipient authentication or session
Ensure SPF, DKIM, DMARC are properly set

Note: For iPost users, 5.5.4 errors are usually triggered by Yahoo/AOL or Microsoft blocking due to content, domain alignment, or sender reputation — not tenant-specific Microsoft limits.


5. Practical Checklist for iPost Users

  • ✅ Authenticate your sending domain: SPF + DKIM + DMARC

  • ✅ Clean your lists: remove inactive users, especially Yahoo/AOL addresses

  • ✅ Avoid risky keywords in subject lines and header text

  • ✅ Segment large campaigns to Microsoft, Yahoo, and AOL recipients

  • ✅ Monitor complaints and unsubscribes (<0.1–0.3%)

  • ✅ Offer clear unsubscribe options (1-click preferred)

  • ✅ Test campaigns using seed accounts

  • ✅ Maintain engagement: send to recently active subscribers only


6. Summary Table: Key Deliverability Considerations

Provider
Focus
Notes for iPost Users
Microsoft (Outlook.com / Hotmail / Live / MSN)
Recipient-based limits & spam filters
Max ~500 recipients per message; high-volume sends may trigger throttling or filtering; ensure SPF/DKIM/DMARC and good engagement
Yahoo / AOL
Bulk sender policies & content filters
Avoid gambling/money-heavy words; authenticate SPF/DKIM/DMARC; maintain clean lists; monitor complaints
All providers
General email hygiene
Segment campaigns, test with seed accounts, monitor engagement, respect unsubscribes

7. Final Takeaways

Deliverability is about trust and reputation, not just sending volume:

  • iPost infrastructure is designed for bulk sending safely

  • Authentication, list hygiene, and engagement are critical

  • Focus on content quality and audience engagement rather than “tricking” filters

By following these practices, your campaigns are more likely to reach inboxes, not spam folders.