SPF and DKIM

SPF and DKIM are two important security methods domain owners have of "authorizing" specific email servers to send mail on their behalf. Besides providing greater deliverability of your messages, these mechanisms prevent fraudsters from sending spoofing emails as your domain.

These TXT records are entered by the domain owner wherever the domain's DNS record is managed, which may be with the registrar or hosting provider. DKIM also adds public key cryptography for deeper validation.

SPF

The following is the most commonly applied SPF record for XMission email customers but it is imperative you understand how and why this is applied. IMPORTANT: It is critical that you read and understand all SPF information below before applying the record to your domain as you could break email delivery.

@    IN TXT "v=spf1 a mx include:_spf.xmission.com ~all"

How It Works

Any SPF record is a string of one more more potential mail sources, prefixed by a character indicating the policy for mail source. An example of a common SPF record:

@     IN TXT "v=spf a mx ~all"

In this case, it says that the A and MX records for the domain are allowed to send, and all other mail fails (but with a "softfail", so that mail isn't actually rejected). "a", "mx" and "all", are all individual mail sources. The "~" prefixed to the "all" source is a policy denoting that mail from the source should be considered a "SoftFail".

The sources are:

• a - A DNS A record
• mx - An MX record
• ptr - A PTR record
• ip4 - An ipv4 address or subnet
• ip6 - An ipv6 address or subnet
• include - The contents of another domain's SPF record
• all - Any mail source (generally used at the end to provide a default policy)

The prefix characters are:

• + - Pass (Valid for SPF)
• - - Fail (Invalid, and reject mail)
• ~ - SoftFail (Invalid, but still accept)
•  ? - Neutral (...whatever...)

Valid SPF for XMission

Any SPF record for a domain sending through XMission should contain "include:_spf.xmission.com".

If you prefer a less identifiable entry then a minimum inclusion would have the following two sources:

• ip4:166.70.13.0/24
• ip4:198.60.22.0/24

Note that the SPF record policy is a decision of the domain owner's. If they want to Fail or SoftFail on all, add other sources, etc., is up to them. We don't have a singular recommendation or requirement for SPF. If the customer only sends via XMission, the following SPF record is relatively safe:

@    IN TXT "v=spf1 a mx include:_spf.xmission.com ~all"

Valid SPF for XMission with additional sending services

SPF Record Merging

Some customers have mailing list or other email sending requirement that require a number of records be strung together in the SPF record. Fortunately the fairly lenient syntax of SPF records makes this easy to do, here is our default record compared to some merged records:

Default XMission: "v=spf1 a mx include:_spf.xmission.com ~all"

XMission plus Mailchimp: "v=spf1 include:_spf.xmission.com include:servers.mcsv.net ~all"

XMission plus Gmail, plus Constant Contact: "v=spf1 include:_spf.xmission.com include:_spf.google.com include:spf.constantcontact.com -all"

Forwarding

A note about forwarding (ie automatically redirecting mail from one email address to another, not hitting the forward button):

Forwarding (under most circumstances) BREAKS SPF. If a domain has a "-all" ("reject all other mail" aka "Fail") policy in their SPF record, mail will be rejected by any servers respecting SPF after the server performing the forward. Users should keep this in mind when choosing a policy. This is why we suggest "~all" ("SoftFail") when you initially configure SPF.

DKIM

How DKIM Works

DKIM is similar to SPF in that it uses a TXT record on the domain to define a sending policy for that domain. Where it differs is that it uses public key cryptography with this sending policy. A public key is added to that TXT record and any message that is signed with the private key (and thus validates with the public key) is then considered valid.

DKIM will break automatic responders, like Out of Office or Vacation replies. This is due to how DKIM verifies the sender, and how automated replies are generated and sent. This is intentional, and is a consideration that needs to be made when limiting sending from your domain via DKIM.

Adding DKIM to an XMission Domain

We have a single private key for XMission DKIM signing. We can sign DKIM with this key on any domain sending out through XMission. It works for Zimbra domains, virtmail, and SMTP relay. To enable this feature for a domain, two things need to happen:

• The domain needs proper DNS for our domainkey key (see below)
• The domain needs to be added to XMission routing config as one with DKIM signing.

DKIM DNS

Add the following two TXT records to the domain:

xmission._domainkey IN      TXT "v=DKIM1; t=y; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCzWmoe0tzQkSUzMqliwcQQ5zY1HKk4z+Wgp+dRCRe7MmSBPftE9r5Lx1QfTfF/J8gl4k9tFsUvUBap0fk1VGMYUG/2LynVuzpkCI4JlUKF5fbx+MDNZrVi0aX73Edjd9trU6NKldVnhNg1RixDLa4aB04XJviy6+3P1h3IHNaZ0QIDAQAB"
_domainkey      IN      TXT "t=y; o=~;"

Multiple Email Hosts with DKIM

It is possible to use more than one host with DKIM, though this is only possible if the DNS records used to announce the public key for the security handshake don't share subdomains with each other.