As long as all the jobs can sync properly then that's fine. The MailAddressResolver classes cannot be tied to jobs, so it's forced to go through the perforce configurations one by one until it finds the user it's looking for (this includes disabled jobs as well, apparently). If a job isn't correctly configured, that could result in a hang (though really it should time out after a while).
In any case, I can't reproduce this behavior at all using 1.0.16 on hudson 1.318, or 1.0.18 on hudson 1.339.
It looks like the reason that it 'worked' in 1.0.15 is because the mail resolver was outright disabled. It looks like another user was running into the same issue, and just commented the entire thing out instead of trying to fix it.
One thing that comes to mind is that while perforce commands are run on the slave during a build, mail resolution actually takes place on the master. If the perforce server is reachable from the slaves, but not the master, then that would produce exactly the behavior you are seeing.