Yeah, I think so. IIRC, Thunderbird has a dialog similar to the one in
Firefox to make it connect through a SOCKS proxy, but I don't know that
any other readers do.
Anyway, AIUI, most Tor users would want to "torify" things as
completely as possible. E.g., even if an app is configured to use
SOCKS, DNS calls will be made in the clear unless forced to go through
something like tsocks.