Most of email clients shows "on behalf", "via" suffixes next to "From" email address if its domain differs from domains of Email Authentications ("Return-Path" email's domain for SPF, "d=" key value for DKIM, for instance). This raises questions:
- What are the rules which should be applied to domain comparison? Top-level domain comparison?
- Is there a standard for this kind of domain comparison?
- Is it up to a particular email client to decide how to compare domains?
Gmail allows to use subdomain (for instance, foo.exmaple.com) in Email Authentication and to have top-level domain (example.com) or even another subdomain (bar.example.com) in "From" field without "Via" information. Do other client behave same way?