[Exim] AUTH PLAIN without a parameter & RFC 2554

Some clients (notable Pine) seem to send "AUTH PLAIN" without a parameter.
It looks like the Pine authors are expecting "334 " in response. (See
http://www.washington.edu/pine/pine-info/2002.03/msg00209.html.) Exim
instead returns "535 Incorrect authentication data" and Pine at this point
gives up. (I've got Pine working on a different set-up with AUTH CRAM-MD5,
however :)

Is this an Exim bug, a Pine (et al) bug, or a clear-as-mud RFC? ;)

Matt