Optimizing PHP website email

i have a website social application used PHP and MYSQL , my hosting is [login to view URL] i have the following problems: 1) Activation emails and other emails sent from my php code on the server can't reach the user's Inbox of ( yahoo, hotmail and gmail ) it reaches to the spam or junk mail. I need a solution , either to fix the php code or to suggest a way to solve this issue. 2) I tried to send Arabic emails to iphone mail client however it is not being displayed properly i need also to fix this problem in the HTML. I need my emails to reach the inbox's of users by any means i am ready to implement any efficient way. Best Regards