Tip: leave the IP blank to auto-detect your current egress IP. Leave the domain blank to skip SPF/DKIM/DMARC checks; try a common DKIM selector first and switch if it isn't found.
Enter an IP to start the check — results will show 12 DNSBLs plus PTR, domain authentication, and ESP detection details.
How this works
Whether mail reaches an inbox comes down to three things: whether the sending IP is on a blocklist, whether reverse DNS is set up correctly, and whether the domain's SPF, DKIM and DMARC records line up. Failing any one of them can get a message rejected or filed as spam.
A DNSBL (also called an RBL) is a publicly maintained list of problem addresses that receiving servers query in real time. Listings usually come from sending spam, belonging to a range that should not be sending mail at all (residential broadband, datacenters), or being reported.
Lists vary enormously in strictness. A hit on one or two lenient lists has limited effect; a hit on a major list effectively stops delivery.
A PTR record resolves an IP back to a hostname, and it is a hard requirement for sending. Most large mail providers reject IPs without one outright, often without explaining why.
Stricter setups require the lookup to match in both directions: the IP resolves to a hostname, and that hostname resolves back to the same IP. A one-way record can still be treated as untrustworthy.
These three are domain-level proof of identity, answering "was this message really authorised by this domain?":
"Can send normally" means no blocking problem was found, though delivery still depends on content and sending history. "Likely to land in spam" usually means a missing PTR or a hit on a secondary list. "Very likely to be rejected" generally means a major DNSBL listing, or a range that is not permitted to send at all.
Most cloud providers' ranges are treated by default as addresses that should not be sending, and some providers block port 25 as well. Before running your own mail server, confirm the provider allows it and request both an unblock and a PTR record.
Most lists offer self-service removal, but you need to fix the cause first — an IP still sending spam gets relisted quickly. On a shared IP, someone else's behaviour affects you too.
Those three are the entry requirement, not a guarantee. Recipients also weigh sending history, content, bounce rates and complaint rates. New domains and new IPs both need time to build a reputation.
Technically yes, practically almost never. Residential ranges are widely listed as not permitted to send, ISPs generally do not offer PTR configuration, and port 25 is usually blocked.