added basic LTMP client class
inline conditional for default arg
convenience arg for Envelope.copy()
allow comparison of replies
put original recipients in bounce message
beginnings of partial relaying support
beginnings of LMTP relayer
more appropriate bounce content-types
allow relays to report partial delivery
implement new function in dict storage
log metadata update in dict storage
pull out common helper func
finish lmtp client relay implementation
remove redundant constructor
allow bounces to be queued separately