i am using Ubuntu operating system
and i have installed zimbra mail server 6.0 package
the installation and configure of mail server was completed

but when i send any mail to gmail or yahoo or any other mail id's
the mail which i have sent is going to spam instead of inbox
but when i send mail from gmail or yahoo i can receive it in inbox.
i am using airtel service provider internet - home plan.

can any one suggest me why this mail is going to spam instead of inbox.

Thanks in Advance.