2 things I see that yahoo thinks its junk.
1) X-YahooFilteredBulk: IP address of server
2) Authentication-Results: mta565.mail.mud.yahoo.com from=domain off ispconfig; domainkeys=neutral (no sig)
For problem 1, I wouldn't know why unless you gave your IP out. Its no big secret since you are emailing from it.
For problem 2, This is because Yahhoo refusees to use SPF records because they sponsor the rival of SPF which is DomanKeys. DomainKeys is known to be buggy and fail when it shouldn't whereas SPF never fails unless its 'admin error'.
My best guess is that your IP block has been tagged for spamming... That doesn't mean YOUR ip is but rather the WHOLE ip block. EG; a /16 or /24 class