I would recommend to check mail logs in /usr/local/psa/var/log/maillog to see what's happening with those email, do they get rejected for some reason or does Yahoo accept them.
I sent an email to my yahoo account from horde. Here is the log:
Feb 12 05:58:21 vps qmail: 1202824701.048453 starting delivery 4504: msg 3286390
62 to remote <deleted>@yahoo.com
Feb 12 05:58:21 vps qmail: 1202824701.048532 status: local 0/10 remote 1/20
Feb 12 05:58:22 vps qmail: 1202824702.001557 delivery 4504: success: 216.39.53.1
_accepted_message./Remote_host_said:_250_ok_dirdel/
Feb 12 05:58:22 vps qmail: 1202824702.001734 status: local 0/10 remote 0/20
Feb 12 05:58:22 vps qmail: 1202824702.001879 end msg 328639062
Nothing shows up in my yahoo, including bulk.