Mail exchanger

A mail exchanger (alternatively known as mail relay, message exchanger or MX, MX host, and SMTP mail exchanger; hereinafter, the App) is a software application that locates DNS records such as MX records and use those records to route emails.

To communicate with mail transfer agents, mail delivery agents, and the other Apps, the Apps use the Simple Mail Transfer Protocol (SMTP).

Records

Main wikipage: DNS record

Several DNS records are used to both evaluate sent mail and map mail transfers.

A record

Main wikipage: A record

Mail exchangers look for A records to locate the IPv4 address of the recipient's host device. An example of this record is

mail.friendsofcnm.org. IN A 134.209.40.232

MX record

Main wikipage: MX record

Any MX record is the DNS record that identifies the server that handles email address for the hostname. MX is an abbreviation for mail exchanger.

Each MX record contains three pieces of information: the hostname, the priority, and the hostname of the mail server that handles mail for the host device. The sample of the MX record is as follows: friendsofcnm.org IN MX 10 friendsofcnm.org, where:

Sample code	Field	Description	Values
friendsofcnm.org	Labels	One or more labels of the hostname and TLD name.	Selected
IN	Class	The Internet or intranet; other options are all outdated.	IN
MX	Record	MX stands for MX record and sets up the relationship between hostname labels and IP address	Stable
10	Priority	A numerical value that signifies the priority of this particular MX record and, consequently, for the mail server. The values used for this are only important if more than one mail server is used. The lower the value of the priority field, the higher the priority of the mail server.	Assigned
mail.friendsofcnm.org	Mail server hostname	The hostname of the mail server that handles email for this domain. This hostname is a google address when Google Apps handle emails for this host device. Any mail server hostname should have a validly configured A record in order to receive emails smoothly.	Assigned

TXT record

Main wikipage: TXT record

Any TXT record is a DNS record that allows for storage of human-readable and machine-readable texts that, if posted, would be assigned to a specific hostname.

With regard to machine-readable texts, TXT records may serve multiple purposes, including:

• Sender policy framework (SPF) data storage. This data confirms the actual systems that are authorized to send mail on behalf of the given hostname. This is useful in the prevention of spam emails being sent with a forged sender address originating from the particular host device. RFC 4408 discourages this practice as "not optimal," however, because SPF now has its own DNS resource record type (code 99);
• DomainKeys Identified Mail (DKIM) data. This data allows a receiving mail server to authenticate entities that have signed a specific email message. DKIM is similar to SPF in that it can help reduce spam email from containing forged email addresses originating from your domain, but it also contains a large amount of additional functionality.

Main wikipage: A record

Mail exchangers look for A records to locate the IPv4 address of the recipient's host device. An example of this record is

mail.friendsofcnm.org. 134.209.40.232

Setting up the MX record

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(There is a ‘dot’ after the domain name) Setting up the SPF record

We create a new TXT record

"v=spf1 a mx ip4:1.2.3.4 -all"

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The SPF record protects from email spoofing. It will simply tell other mail servers that only our server is authorized to send emails for yourdomain.com (more aboutSPF). Setting up the DMARC record

We create a new TXT record named _dmarc.yourdomain.com. (There is a ‘dot’ after the domain name)

"v=DMARC1; p=quarantine; rua=mailto:postmaster@yourdomain.com"

Now we will setup the hostname for the PTR record

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It will take a while to propagate the new configuration throughout the entire internet.

PTR record

Main wikipage: PTR record

Any PTR record is a DNS record that translates a hostname into an IP address. PTR is an abbreviation for pointer; PTR records point to IP addresses. In comparison with A records, PTR records perform the exact opposite function.

PTR records use the following format: <IP address in a reverse order>.in-addr.arpa PTR <hostname>. For instance, 1.93.89.159.in-addr.apra PTR friendsofcnm.org, where:

Sample code	Field	Description	Values
1.93.89.159	Reversed IP address	The IP address of the location that the resulting hostname points to in a reverse order. The actual IP address used in this example is 159.89.93.1	Assigned
.in-addr.arpa	Domain	The domain name that historically arrived from the times when the Internet was called Arpa. In-addr is an abbreviation for internet address.	No other options
PTR	Record	PTR stands for PTR record and sets up the relationship between IP address and hostname.	Stable
friendsofcnm.org	Hostname	The hostname that points to the IP address.	Selected

PTR records are needed for outgoing mail servers such as Postfix, because most of the mail providers reject or mark as spam messages received by mail servers without valid reverse dns configuration such as a missing PTR record or mismatch with an A record for the hostname.