After adding the external support address to Zendesk Support, the email address is not verified. I see the error message Forwarding check failed.
Verify that the forwarding rules within your email domain are set up correctly. The verification error is due to a ping that is sent from the Support account to the linked support address. If forwarding is set up correctly, that ping returns back to the account. For more information on forwarding your email to Zendesk, see the article: Forwarding incoming email to Zendesk Support.
If the forwarding is set up correctly in your account, but you receive an error for the SPF record check, ensure that there is an SPF record set up for that address.
The SPF record is not required for Zendesk, to send outgoing notifications as the system simply sends out notifications from a Zendesk server. For more information, see Allowing Zendesk to send email on behalf of your email domain.
After you completed the steps mentioned, if you still see the error message, we recommend reaching out to your mail admin or email provider for further assistance.