Murray S. Kucherawy writes:
I think having a message with User-Agent and List-Agent is less confusing than one with two User-Agents.
Who's going to be confused? Not end users.[1] I would think the real application is for an administrator or software to look at them and go, "Uh-oh, it's another Outlook|fml|... message" and deal with that agent's peculiarities. As long as there's only one set of foibles associated with Outlook, and another with fml (perhaps not disjoint from Outlook and perhaps associated with several fml-wannabes as well, but consistent across those MLMs that emulate fml), you probably don't need to distinguish -- "MLM" vs. "end-user client" is something (ie, a foible) you can deduce from the agent's name and version string.
While I'm not terribly happy with calling an MLM a *user* agent, it's not entirely farfetched, especially in the case of something like Mailman. First, the MLM functions as an agent for the list owner, filtering out evile posts and protecting archives from the eyes of the unwashed. Second, it functions as the agent of each subscriber, avoiding dupes, suspending delivery during vacations, digesting the posts, etc.
Footnotes: [1] As the late great George Carlin's alter ego Al Sleet the Hippy Dippy Weatherman put it, "It's 29 degrees at the airport ... which is stupid, 'cause nobody I know lives at the airport." And nobody I know (except me) looks at User-Agent/X-Mailer headers.
The-exception-that-proves-the-rule-ly y'rs,