If you send email from your server, a PTR record is not a detail, it is a requirement. Receiving mail servers check whether your IP has a matching reverse DNS entry, and otherwise tend to reject the message or move it to spam. At ComputeBox you set the PTR right in the panel. This guide shows how it all fits together.
6 minBeginnerTested on ComputeBox panelUpdated 2026-06-17
In short
Point forward DNS (A/AAAA) at a hostname, enter the same hostname as the PTR in the panel under Networking, then verify with dig -x.
A normal DNS record resolves a name to an IP, for example mail.example.com to 91.108.80.108. A PTR record does the opposite: it resolves the IP back to a name. Mail servers use this as an authenticity check. The key is that both directions match: the PTR points to a hostname, and that hostname points back to the same IP via an A or AAAA record. This is called forward-confirmed reverse DNS.
Open the Networking tab in the panel. Your IPv4 and your IPv6 network are listed there, each with a PTR button. Click PTR on the IPv4 and enter exactly the hostname you just created:
If you send mail over IPv6, set the PTR for the specific IPv6 address your mail server sends from too. You get a whole /64 network, so the PTR belongs on the address actually used, not on the entire network.
Reverse DNS can take a few minutes. Then check from your own machine:
The output should show your hostname:
If the name matches, the PTR is active. Without dig, host 91.108.80.108 works too.
Port 25 for outgoing mail
Outgoing port 25 is blocked by default at ComputeBox. For your own mail server we unblock it per server after a quick check, just open a ticket. Ports 465 and 587 are open.
No. For web, APIs, or databases a PTR is not needed. It mainly becomes a requirement once you send email yourself.
What happens without a matching PTR when sending mail?
Many receiving servers reject mail from IPs without a valid reverse DNS entry or flag it as spam. A matching PTR is one of the basic requirements for deliverability.
Does the PTR have to match the forward DNS exactly?
Yes, that is the whole point. The PTR points to a hostname, and that hostname must point back to the same IP via an A or AAAA record.
