When you send email from a dedicated IP, your deliverability depends on more than just your own sending behavior. Gmail, Outlook, and Yahoo also evaluate the other 253 IP addresses in your /24 block — your "neighbors."
A /24 is a block of 256 consecutive IP addresses (e.g., 149.72.152.0 through 149.72.152.255). This is the standard unit that ISPs, blacklist providers, and email security systems use when evaluating IP reputation.
When Gmail receives an email from a new IP, it has no sending history to evaluate. So it looks at context signals — and the most accessible one is: what else is happening in this /24 block?
If your IP sits in a block where 200 other IPs are sending legitimate email from recognizable brands, that's a positive signal. If your IP sits in a block where 250 IPs have no reverse DNS and 3 are on Spamhaus SBL, that's a negative signal — even if your specific IP is clean.
This is why two identical email campaigns, with identical content and identical authentication, can have different inbox placement rates on day one. The IP neighborhood is the hidden variable.
High occupancy. A block where 240+ of 254 IPs have active PTR (reverse DNS) records means the block is in active use by real senders. ISPs have reputation data on this range.
Custom PTR records. When a sender configures custom reverse DNS (e.g., mail.shopify.com instead of o12345.outbound-mail.sendgrid.net), it signals a serious, high-volume sender who has invested in their email infrastructure.
Zero blacklist hits. Especially no Spamhaus SBL listings. A single SBL listing in a /24 can cause ISPs to throttle or junk mail from the entire block.
Healthy adjacent blocks. Some anti-spam systems (notably Spamhaus) evaluate at /23 granularity — meaning the /24 block next to yours also matters.
Ghost blocks. A /24 where most IPs have no PTR record is a "ghost block." Nobody is sending from it. ISPs have no reputation baseline, and warmup from a ghost block takes significantly longer.
Blacklist contamination. Even 2-3 IPs on Spamhaus SBL can poison the entire /24. The blacklist applies to the individual IP, but ISPs that see SBL listings in a block become suspicious of all traffic from that range.
Mixed-use VPS blocks. Vultr, DigitalOcean, Linode, and Hetzner blocks are shared with web servers, VPNs, and occasionally malware. These blocks tend to have higher blacklist rates and no ESP reputation signal.
Enter any IP address in the scanner above. We'll resolve PTR records for all 254 IPs in the /24, check each against 10 major blacklists, identify known senders, scan adjacent blocks, and produce a scored report with specific recommendations.
Free. No signup. Full audit of 254 IPs in under 60 seconds.