Best setup / method for sending mail from a PHP application ?

OK, so - I am using a PHP application which needs to send emails to users such as welcome messages, forgot passwords, password changes, newsletters etc.

I have configured Postfix successfully by following: https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-postfix-as-a-send-only-smtp-server-on-ubuntu-14-04 .

I have Google apps set up for my domain to receive incoming email, which is working as expected

My server sends out the emails just fine, however some recipients such as Gmail for example will send the emails straight to spam.

1. If possible, could someone please explain the steps for making sure emails sent out out by my server are not marked as spam by recipients?

2. Would it be a better option to connect the PHP to a 3rd party service such as Mailgun or similar to send outgoing mail, and if so, is this difficult to do?

Also, I had considered using SMTP to access my Google app email account to send email but there is a limit of 100 every 24 hours so this is not viable.

I am sure there is a more experienced developer/sysadmin who has been through this setup before and I would greatly appreciate the help.

Thanks,

• Dave