Update:
This is posted to mailbox.log on zmail1 when an IM user on zmail3 requests to add a zmail1 user and the zmail1 IM user selects the "Accept and Add" on the IM popup:
2008-10-03 12:45:52,660 INFO [btpool0-419] [name=blaze@zmail1;mid=22;ip=10.3.2.249;ua=ZimbraWe bClient - FF3.0 (Win)/5.0.9_GA_2533.RHEL5_64;] im - IMSubscribedNotification blaze@zmail3 Subscribed=false Ask=subscribe
2008-10-03 12:45:52,660 INFO [btpool0-419] [name=blaze@zmail1;mid=22;ip=10.3.2.249;ua=ZimbraWe bClient - FF3.0 (Win)/5.0.9_GA_2533.RHEL5_64;] im - IMSubscribedNotification blaze@zmail3 Subscribed=false Ask=subscribe
2008-10-03 12:45:52,666 INFO [pool-5-thread-1] [] im - IMPersona(blaze@zmail1 ONLINE pri=1 st=null) - Presence.error: <presence to="blaze@zmail1" from="blaze@zmail3" type="error"><error code="404" type="cancel"><remote-server-not-found xmlns="urn:ietf

arams:xml:ns:xmpp-stanzas"/></error></presence>