Google and Yahoo tightened bulk sender requirements over the past couple of years, and by now most major inbox providers expect senders to have proper email authentication in place. If you're running an Email Marketing VPS, here's a plain-language breakdown of the three records that matter.
SPF (Sender Policy Framework)
SPF is a DNS record listing which servers may send email on behalf of your domain. Without it, receiving mail servers have no way to confirm your VPS is actually authorised to send as you.
DKIM (DomainKeys Identified Mail)
DKIM adds a cryptographic signature to outgoing messages, letting the receiving server verify the message wasn't altered in transit and genuinely came from your domain.
DMARC (Domain-based Message Authentication)
DMARC tells receiving servers what to do when a message fails SPF or DKIM — reject, quarantine or deliver — and gives you visibility into who is sending mail as your domain.
The fourth record nobody lists
A PTR record — reverse DNS on your sending IP — is checked by a great many receiving servers, and mail from an address with no PTR is treated with suspicion regardless of how good your SPF and DKIM are. We set PTR records on request, and the VDS range states PTR/rDNS support in the plan itself.
Getting started
If you're setting up bulk sending on a VPS, configuring all four correctly — along with a clean sending IP and a sensible volume ramp-up — has a much bigger effect on deliverability than which sending software you choose. Our Email Marketing VPS guidance covers the sizing question, and Port 25 is open on every plan and guaranteed on the Cloud and VDS ranges. If losing the ability to send would stop your business, buy the guarantee rather than the default.