If you are not able to send email from your server / VPS OR the email sent from your server are going to Junk or Spam Box; Following are some Important Settings and configurations need to be checked before you contact Yahoo, Hotmail, AOL

1) RDNS

Reverse DNS (rDNS or PTR) is a process of resolving an IP address into a domain name, just like a domainname (DNS Propagated) resolves to IP addresses.
You can check if RDNS for your Server/VPS is set of not using following command from the Shell of Linux Machine

dig +short -x xxx.xxx.xxx.xxx

Where xxx.xxx.xxx.xxx will be IP of your Server. You can also check RDNS using tools provided by DNS sites like http://iptools.com/
Please contact you Webhosting Company to set RDNS or PTR record for the Server or VPS IP

2) SPF

SPF (Sender Policy Framework) is a system to help domain owners identify all the servers which are expected to send mail from their domain. The aim is that other mail systems can then check to make sure the server sending email from that domain is authorized to do so. This helps to reduce the chance of email ‘spoofing’, phishing schemes and spam. SPF record need to be added in the DNS zone of the domain from the server DNS for the domain is resolving.

To add SPF record open the zone file of the domain and add following

<domainname.com> 14400 IN TXT “v=spf1 a mx ~all”

3) Domain Keys

DomainKeys is an e-mail authentication mechanism designed for verifying the identity of an email sender. It is one of cryptographic authentication method to verify sender’s identity and the integrity of email content. It also verify DNS records of domain of an email sender and the message integrity. DomainKeys has implemented standard of Identified Internet Mail to create enhanced protocol called DomainKeys Identified Mail (DKIM). You can read more about DomainKeys here.