Troubleshooting SMTP for guest registeration

[SMD28316]

I have ISE 2.7 and testing SMTP for Self-registration with sponsor approval, when a guest create their own account they receive this error:

" Your request has been forwarded to a sponsor. The credentials will be sent to the e-mail indicated in the registration form The request for an account must be approved. but the sending of the email message with the request failed or the SMTP extension is not configured correctly. Try registering again or check the SMTP configuration."

What could be the issue? I have verified the SMTP configuration multiple times, 

also, I would like to take a packet capture from Cisco ISE to check the SMTP messages, should I collect them from the PSN handling the guest authentication or from the primary PAN node?

[balaji.bandi]

check the SMTP config and test :

https://www.cisco.com/c/en/us/support/docs/security/identity-services-engine/216187-configure-secure-smtp-server-on-ise.html

[SMD28316]

yes I read this document, but where do I collect the captures from? the PSN or the primary admin node?

[balaji.bandi]