Dynamics 365 Online with Exchange on-premises
Verify your firewall allows communication from Dynamics 365 online. For a list of the IP address ranges that may be used by Dynamics 365, see Microsoft Dynamics CRM Online IP Address Ranges
POP3/SMTP Providers such as Gmail
Verify the correct ports are specified:
1. Navigate to Settings, Email Configuration, and then click Email Server Profiles.
2. Open the Email Server Profile that is used by the mailbox encountering this error.
NOTE
You can also click the name of the Email Server Profile within the error message. This appears as a hyperlink which will open the correct Email Server Profile record.
3. Click Advanced.
4. Verify the Incoming Port and Outgoing Port values and then click Save.
NOTE
Refer to the documentation from your email provider for the correct ports to use when connecting to the SMTP service. For Gmail the incoming POP3 port is 995 and outgoing SMTP port is 587.
5. Click Mailboxes and then select the mailbox that encountered the error.
6. Click the Test & Enable Mailboxes button.
7. After the tests complete, open the mailbox record and view the Alerts section if the results do not appear as Success.