Learn why Contact and Lead assignment notifications may not be delivered to the new owner

To verify your organization can receive email from every Salesforce IP address

1. Click Setup | Email Administration | Test Deliverability in Salesforce Classic or Setup | Email  | Test Deliverability in Lightning Experience. Review Test the Deliverability of Emails Sent Through Salesforce.
2. Enter your business email address.
3. Click Send. Salesforce will simultaneously send a test message from all IP addresses to your business email address. Each test message specifies the IP address from which it was sent.
4. Check your business email account to make sure it received all test messages.

Notes: 

• If you do not receive the test messages, your organization's email administrator must allowlist the Salesforce IP ranges on your organization's email server. Please review Ensure you can receive email from the Salesforce Application to learn which IP addresses to allow list.
• If you have administrator rights, you can generate the email logs to determine if there was a delivery failure, and what the error message was. To learn how, please review Use Email Logs to Monitor Emails Sent from Salesforce and Email Log Reference.