Disadvantage of using IP address for auditing
1.1 Hard to know which IP belongs to a server, firewall, appliance, etc..
1.2 Master list on an excel file is not a best practice where multiple copies can be created.
Advantage of applying alias instead for SMTP Auth
2. Any updates on the alias, we no longer have to add or delete in the configuration document.
2.1. Checking on aliases can be directed to the system owner of DNS for checking and verification. No longer have to send memo to business units, IT developers and other IT units.
Note: Environment that have about thousands of servers (prod, dev, uat and sit environments) where most of them would require SMTP relay access. And SMTP is one of many ways a malware can exploit if not planned properly.