1. MX record
A mail exchanger record (MX record) is a type of resource record in the Domain Name System that specifies a mail server. An MX record is the fully qualified domain name of a mail host and a preference value. The host name must map directly to one or more address record (A, or AAAA) in the DNS, and must not point to any CNAME records.
Sender Policy Framework (SPF) is a simple email validation system designed to detect email spoofing by providing a mechanism to allow receiving mail exchangers to check that incoming mail from a domain is being sent from a host authorized by that domain's administrators. The list of authorized sending hosts for a domain is published in the Domain Name System (DNS) records for that domain in the form of a specially formatted TXT record. Email spam and phishing often use forged sender addresses, so publishing and checking SPF records can be considered anti-spam techniques.
DomainKeys Identified Mail (DKIM) is an email validation system designed to detect email spoofing by providing a mechanism to allow receiving mail exchangers to check that incoming mail from a domain is authorized by that domain's administrators and that the email (including attachments) has not been modified during transport. A digital signature included with the message can be validated by the recipient using the signer's public key published in the DNS.
DKIM Validation Tools:
4. rDNS reverse DNS Resolution：
An e-mail Anti-spam technique is to check the domain names in the rDNS to see if they are likely from dialup users, dynamically assigned addresses, or other inexpensive Internet services. A recent shift has shown that spamming has switched to mainly coming from hosting companies making using rDNS even less useful. All of this adds to the argument that the few services that choose to block email servers purely on the basis of rDNS are simply discriminating without merit and often miss out more pro-active and useful indiscriminate anti spam measures.