[Koha] Email configuration reg

Respected Sir, Iam Chitti Babu B,Librarian,J.H.A Agarsen College,Chennai,Tamil Nadu,India.i had installed Koha latest version on Linux Ubuntu.i wanted to send email alerts to my patrons for books which are overdue.we don`t have an email server.We installed POSTFIX.We tried to configure email in our server.It says email sent to patron.Bit when we checked in th inbox and spam there is no email.Source mail id is created for Library using GMAlL.Recepient email is also GMAIL.I need your help.Kindly guide to fix the problem.Also tell the proper steps to be followed for email configuration. Thanking You. Yours truly, B Chitti Babu.