Reaching someone's inbox is definitely getting harder. Even if you have taken the steps to avoid your email being classed as spam. You now need to prove you are a legitimate sender and not sending or impersonating a fake domain. Many of the major email providers (Gmail, Hotmail, Yahoo) are now expecting these extra levels of 'authentication'.
Here are the necessary steps to dramatically improve the success of email delivery.
A very basic recommendation is that a reverse DNS lookup of your IP address should match the domain name you are sending email from.
Sender Policy Framework (SPF)
An SPF record is a TXT record which is added to your DNS. This record outlines which hosts/IP addresses are allowed to send emails from your domain name. When a email server receives an email, it looks at this and checks to see if the sender matches one of the entries in this record.
DomainKeys Identified Mail (DKIM)
DKIM is a way of digitally signing an email from a domain. It's more involved to setup, but more trusted. You must be sending emails from a SMTP server which supports DKIM. The basic steps are to create a public/private key, get your SMTP server to sign the emails with the private key, and add DNS records to your domain with the public key details. When an email server receives an email which has been DKIM signed, it checks to see if it matches correctly.
Testing email authentication
Port 25 have a great email authentication tool where you can send them an email and it'll email you back with a report on how well the email authenticates.
Airsquare customers already setup
If you are an Airsquare customer, and we manage your domain for you, all these steps have already been completed for you. If you look after your own domain, give us a shout and we'll walk you through.