ISPconfig that I am using cannot send e-mail to Yahoo Inbox, instead it will be at Bulk folder. For ISPConfig 2.2.6, could the e-mail delivered to Yahoo Inbox?
Thats not a question of ISPConfig, as ISPConfig does not send emails. Emails are send with your MTA, thats either postfix or sendmail. But I guess your Yahoo problem is either DNS related or you send mail from a DSL line. Mke sure you created SPF records for your domain and that you have reverse DNS records for the IP of your server that resolve to the domain of your server.