There are various hints on the internet that the email addresses from the SMTP System Alias Table can be found in QUSRSYS/QATMSMTPA. It is not at all clear to me though if IBM supports a user directly accessing this file as it’s purpose is not documented in the Information Center. The file is however externally described, so who knows.
The more traditional approach (which is clearly supported by IBM) is to use the System Distribution Directory (SDD) to store email addresses. This directory can be searched using the Search System Directory API QOKSCHD in order to obtain the email address. In addition you can also publish e-mail information using LDAP, etc when they are in the SDD. In order to convert addresses from the System Alias Table there is the Convert SMTP Names command CVTNAMSMTP.
Hope this helps,