When connecting with IPv6 from a host with no reverse DNS, evo will send the IPv6 literal in the EHLO without properly surrounding it in []. It looks like camel_gethostbyaddr() is actually returning "2002:c1ed:....:a42a" as if it were the host name, and Evo then just says "EHLO 2002:....:a42a". And gets told to sod off by the mail server, because that violates RFC2822.
Will be fixed in next build.