> For the complete documentation index, see [llms.txt](https://kasplo.gitbook.io/kasplo-doc/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://kasplo.gitbook.io/kasplo-doc/product-docs/welcome-to-the-support-documentation/email-sending-domain-whitelisting-guide.md).

# Email Sending Domain Whitelisting Guide

### 1. Overview

Sometimes, emails sent from a legitimate domain may be rejected or bounced by recipient mail servers. Common causes include spam filtering, security policies, or domain reputation issues.

Purpose of this guide:

* Explain why domain-related bounces occur
* Show how to whitelist sending domains in common email platforms
* Provide general steps to request domain delisting

### 2. Why Domain-Side Bounces Happen

1. Domain Not Whitelisted\
   Some organizations allow emails only from trusted domains. Non-whitelisted domains may be rejected.
2. Strict Spam Filtering\
   Emails from unknown domains can be blocked due to:
3. New domain or low sending volume
4. Low IP/domain reputation
5. Missing email authentication
6. Email Authentication Failure\
   Ensure the following are properly configured:
7. SPF – Authorizes servers to send emails for the domain
8. DKIM – Ensures message integrity
9. DMARC – Defines actions for unauthenticated emails
10. Domain or IP Reputation Issues\
    Previously flagged domains or IPs may be blocked automatically.
11. Organizational Security Policies\
    Some companies maintain strict inbound email rules requiring explicit approval.

### 3. Whitelisting Sending Domains

#### Gmail (Google Workspace)

Documentation:[ Allowlists and Approved Senders](https://knowledge.workspace.google.com/admin/gmail/advanced/allowlists-denylists-and-approved-senders)

Steps:

1. Log in to Google Admin Console
2. Navigate to: Apps → Google Workspace → Gmail → Spam, Phishing and Malware
3. Click Email Allowlist → Add
4. Enter the sending domain (e.g., example.com)
5. Save changes

#### Outlook / Microsoft 365

Documentation:[ Tenant Allow/Block List](https://learn.microsoft.com/en-us/defender-office-365/tenant-allow-block-list-about)

Steps:

1. Log in to Microsoft 365 Defender Portal
2. Navigate to: Email & Collaboration → Policies & Rules → Threat Policies
3. Open Tenant Allow/Block List
4. Select Allow
5. Add the sending domain
6. Save changes

#### Zoho Mail

Documentation:[ Spam Control Lists](https://www.zoho.com/mail/help/adminconsole/spam-control-lists.html)

Steps:

1. Log in to Zoho Mail Admin Console
2. Navigate to: Security & Compliance → Spam Control → Allowed List
3. Add the sending domain
4. Save changes

### 4. General Steps to Delist a Sending Domain

#### Step 1 – Verify Email Authentication

Ensure SPF, DKIM, and DMARC are properly configured.

#### Step 2 – Check Domain and IP Reputation

Tools to check reputation:

* MXToolbox
* Talos Intelligence
* MultiRBL lookup

#### Step 3 – Review Bounce Messages

Identify the reason for email rejection from SMTP bounce codes:

* 550 Unauthenticated mail not allowed
* 554 Message rejected due to spam content
* 421 Temporarily deferred due to reputation

#### Step 4 – Submit Delisting Requests

Most providers offer a delisting process:

* Microsoft SNDS / Delisting Portal
* Google Postmaster Tools
* Spamhaus Removal Center

Include:

* Sending domain and IP address
* Sample bounce messages
* Issue description

#### Step 5 – Maintain Best Practices

* Send emails only to opt-in recipients
* Monitor bounce rates and engagement
* Keep mailing lists clean
* Monitor domain/IP reputation
* Implement email authentication

### 5. Conclusion

Whitelisting your sending domains helps prevent bounces and ensures successful delivery. Combine whitelisting with proper authentication and reputation monitoring for best results.<br>

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If any support is needed, contact: [HelpDesk | Kasplo](mailto:helpdesk@kasplo.com)
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