End to end reliability is accomplished by having the agents remember
 outbound messages until they have been explicitly acknowledged by the other
 side. Messages have sequence numbers to make explicit ACKs possible, which
 also prevent replay attacks. Transport-level NAKs (like bounce messages if
 we're lucky) can speed up retransmits but are not required.

 Recipients (like those behind remailers) can opt out by publishing a flag in
 their IDRecord to say that they don't send ACKs, and individual messages can
 have a flag to indicate whether an ACK is expected or not, good for mailing
 lists.