SMTP mail can come from a PAN, Mnt or PSN node, depending on the function or feature invoking an email.
Guest related email most commonly comes from the PSN although Guest expiry notices (when configured) would come from the PAN.
Reference:
http://www.cisco.com/c/en/us/td/docs/security/ise/2-0/installation_guide/b_ise_InstallationGuide20/Cisco_SNS_3400_Series_Appliance_Ports_Reference.html