Every IP address lives in a block of 256 consecutive addresses called a /24 subnet. When Gmail, Outlook, or any major mail provider evaluates an email from your server, they don't just look at your specific IP — they look at the block you're in. The senders sharing that block with you directly affect whether your emails reach the inbox or land in spam, before you've sent a single message.
This is called the neighborhood effect, and it's one of the most under-documented causes of email deliverability problems.
An IPv4 address looks like this: 149.72.152.42. The first three numbers (149.72.152) identify the /24 block. Every IP from 149.72.152.0 to 149.72.152.255 is in the same /24 neighborhood — 256 addresses, of which 254 are usable.
When you get a dedicated IP from SendGrid, Mailgun, Postmark, or any other ESP, you're assigned one of those 254 addresses. The other 253 are your neighbors. Some are sending transactional email for legitimate businesses. Some are ghost IPs with no reverse DNS. Some, in poorly managed blocks, may be on spam blacklists.
You don't choose your neighbors. And your neighbors affect your reputation.
Google's official sender guidelines confirm that shared IP reputation contaminates all senders in a block:
"A shared IP address is an IP address used by more than one email sender. The activity of any senders using a shared IP address affects the reputation of all senders for that shared IP address."
— Google Workspace Admin Help [1]
This applies even to dedicated IPs — because what Google can't verify about the IPs next to yours, it infers from the block's overall behavior pattern.
Microsoft's behavior is more explicit. The Outlook.com Postmaster documentation describes bounce code S3150:
"Unfortunately, messages from [IP] weren't sent. Please contact your Internet service provider since part of their network is on our block list."
— Microsoft Outlook.com Postmaster [2]
The phrase "part of their network" is key. Microsoft is not saying your IP is blocked — it's saying the network block containing your IP has problems. This is /24-level blocking.
Spamhaus is the most widely used email blacklist in the world. Their SBL documentation states they may apply "escalated SBL listings to extended ranges of that network" when a network is deemed a threat.[4]
More significantly, Spamhaus explicitly uses the term "IP neighborhood" in their reputation framework:
"What is the IP neighborhood... All this signal is analyzed to indicate if, when, and how users should engage and take action."
— Spamhaus IP Reputation [5]
"It is commonly accepted that /24 IPv4 address spaces (256 hosts) are assigned to organizations, and among these networks, one or several IPs are used to send emails. Blocking lists and reputation lists: Single IPv4 addresses and sometimes /24 networks are listed in blocking and whitelists."
— LinkedIn Engineering [7]
The ivmSIP/24 blacklist exists specifically to block IP ranges, not individual IPs:
"Listing in the ivmSIP/24 blacklists indicates that your IP address has been identified as a spam-sending server or is in a range of IP addresses of a host-provider that includes spam-sending servers. Subnet-based Blacklists are used to reject email from entire ranges of IP Addresses."
— MXToolbox [8]
Your IP can be listed on ivmSIP/24 even if you've never sent spam — simply because enough of your neighbors have.
"Level 2 escalates to /24 network ranges when 'too many' Level 1 listings appear in that range. Your IP can be listed even if you've never sent spam."
— Email Almanac [9]
When you get a new dedicated IP, you need to warm it up — gradually increase sending volume so ISPs can build a positive reputation signal. This typically takes 4-8 weeks.
What most warmup guides don't mention: the block you're in dramatically affects how that process goes.
Postmark's engineers put it plainly:
"ISPs and blacklists will monitor entire IP ranges and domains. If one IP causes enough problems, traffic from the entire subnet or domain could be blocked."
— Postmark [11]
Mailjet frames it with a neighborhood analogy:
"IP addresses can be grouped into IP networks and the reputation of the whole network can directly impact your individual IP reputation. Think about it this way: if your IP address is like your home address, then your IP network is your neighborhood."
— Mailjet / Sinch [12]
A ghost block is a /24 where most IPs have no PTR records — no reverse DNS configured. ISPs have no reputation context. Warming up in a ghost block means starting with zero positive neighborhood signal — you're not in a bad neighborhood, you're in an invisible one.
A block populated by high-volume legitimate senders gives ISPs positive contextual signal about your IP before you've sent a single message. The M3AAWG, the industry standards body for email infrastructure, describes the mechanism:
"Sharing an environment between more than one entity means that the reputation for the IP is also shared... the poor sending or list-building practices of one entity can 'poison' the environment, impacting other entities provisioned in the environment."
— M3AAWG Sender Best Practices [13]
The same mechanism works in reverse — good neighbors provide positive context.
You provision a new server. Your IP looks clean, but the /24 had a spam operation three months ago. The individual SBL listing expired, but the UCEPROTECT Level 2 listing for the whole /24 is still active. Your password reset emails go to spam.
"The entire 198.74.52.0/22 subnet has been blocked by Microsoft and is also on the UCEPROTECT black list... This causes large numbers of Linode's IP addresses to be blocked; adversely impacting us real customers who are not sending spam."
— Linode Community [14]
You upgrade to a dedicated sending IP. Three weeks into warmup, inbox placement on Gmail is 40%. Everything checks out — authentication, content, blacklists. But the /24 has 240 of 256 IPs with no PTR records at all. A dead zone with no reputation signal.
Your app runs on Railway, Render, or Fly. You're routing transactional email through the platform. One neighbor runs aggressive campaigns and gets the block flagged. Your emails start going to spam. You didn't change anything.
Check before you accept. Before warming up any new IP, scan its /24. Check occupancy, PTR records, blacklists, and adjacent blocks. If the block scores poorly, release and request a new one.
Request a new IP on a VPS. Most providers will reassign for free. Scan the new one before you start using it.
Separate transactional from marketing email. Route through an ESP that manages neighborhood quality rather than through your platform's native mail server.
Monitor adjacent blocks. Some anti-spam systems evaluate at /23 granularity. Checking the blocks above and below yours costs nothing.
All 254 IPs checked against 10 blacklists. PTR records, adjacent blocks, neighborhood score. Free.