• Sending e-mail is easy but making sure the recipient gets it is not.

Reverse DNS

• Make sure whoever issues your IP has their "reverse DNS" records correct.
• The article mentions "ISP," but this is also whoever you buy your virtual machine access from.
• This is also known as a "PTR" record.

SPF

• Mostly a microsoft/hotmail thing.
• Add a TXT record to your DNS which identifies servers allowed to claim they are delivering mail for you.

v=spf1 a mx ip4:10.0.0.1 ip4:10.0.0.2 ~all

• ip4:

DKIM

• DKIM signs messages with a secret key to prove it was started from an authorized server.

Generate them with OpenSSL:

$ openssl genrsa -out rsa.private 1024
$ openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM

Add some more TXT records to your DNS:

_domainkey.example.com "o=~; r=contact@example.com"
selector._domainkey.example.com "k=rsa; p={public-key-base64-string-here}"