IP reputation

IP reputation is a per-IP score that email providers maintain. It influences whether messages from that IP get inbox placement, promotions placement, or spam folder. Understanding shared vs dedicated IPs and how reputation builds changes how you architect sending.

How IP reputation is built

Gmail, Outlook, and others track every sending IP's:

• Volume over time (sudden spikes look spammy)
• Bounce rate (high bounces = bad list)
• Complaint rate (high complaints = spam)
• Engagement (opens, replies, forwards)
• Unsubscribe rate
• Content patterns
• Authentication pass rate

The score is invisible to you. What you see: inbox placement dropping, spam rate rising, Postmaster Tools showing reputation at "Low" or "Bad."

Shared vs dedicated IPs

Shared IP (default for most cold email)

When you use Google Workspace, Microsoft 365, or most cold email tools, your outbound traffic goes through IPs shared with other senders. The provider manages the pool.

Pros:

• No setup
• Reputation is established (assuming the pool is well-managed)
• Lower volume per sender = less likely to hit rate limits

Cons:

• Your reputation is partly tied to other senders in the pool
• If a major spammer uses the same pool, you suffer
• You have no direct control

For Google Workspace mailboxes sending cold email, you're on shared IPs and generally that's fine. Google's pool management is strong.

Dedicated IP (SMTP providers, high volume)

Paid SMTP providers (SendGrid, Mailgun, SES with dedicated IP) give you an IP used only by you. Your reputation is entirely your own.

Pros:

• Your reputation is purely a function of your behavior
• No contamination from other senders
• Higher volume ceilings

Cons:

• Starts with zero reputation, must warm like a new mailbox
• Requires sustained volume to maintain reputation (low volume = slow degradation)
• Higher cost
• More operational complexity

Dedicated IPs make sense when you're sending 50K+ emails/day from one system. For typical cold email operations (1000-5000/day across 10 mailboxes), shared IPs are usually fine.

The cold email reality in 2026

Most successful cold email operations use:

• Google Workspace or Microsoft 365 (shared IP, managed by provider)
• Multiple mailboxes across multiple domains
• Volume capped at ~30-50/day per mailbox
• Warming running continuously

This approach doesn't need dedicated IPs. The complexity would outweigh the benefit.

IP reputation vs domain reputation

Both exist. They're related but distinct:

• IP reputation: about the sending server
• Domain reputation: about your from-domain

Gmail weights domain reputation heavily. Outlook/Microsoft weights IP reputation heavily. Both matter. Bad behavior damages both.

How to check IP reputation

• Google Postmaster Tools: shows IP and domain reputation for Gmail
• Microsoft SNDS (Smart Network Data Services): shows IP reputation for Outlook/Hotmail
• Talos Intelligence: Cisco's database, shows IP reputation
• MXToolbox blacklist check: shows if the IP is listed on spam blacklists
• Spamhaus: the most-respected spam blacklist database

Blacklists

Being on a blacklist means some providers will reject or filter emails from that IP or domain. Major blacklists:

• Spamhaus (SBL, XBL, PBL, CSS)
• Barracuda
• SpamCop
• Invaluement
• SORBS

If you end up on a blacklist, follow the de-listing process for that service. It's usually free but slow (days to weeks). Prevention is much cheaper than recovery: maintain low bounce rates, low complaint rates, and clean lists.

How to protect IP reputation

The list is short:

• Verify every email address before sending
• Keep bounce rate under 2%
• Keep complaint rate under 0.1%
• Remove unengaged recipients after ~3-6 months of no interaction
• Honor unsubscribes immediately
• Don't buy lists of unknown quality
• Don't send to role-based addresses (info@, sales@) that often get complained about
• Don't send from new IPs/domains without warming
• Maintain consistent volume (don't spike)

When IP reputation damages you

Signs your IP reputation is suffering:

• Open rates dropping week over week with no copy change
• Reply rates dropping
• Postmaster Tools showing "Low" or "Bad" reputation
• Bounces climbing even though list quality is same
• Spam folder placement rising (check mail-tester or seed addresses)

Actions:

1. Pause cold sending for 1-2 weeks
2. Continue warming to rebuild reputation
3. Resume at 30-50% of previous volume
4. If no recovery in 4-6 weeks, switch to new sending domains and mailboxes

The alternative: retire bad infrastructure

If a domain or IP has been badly damaged, the fastest path forward is usually not to fix it, it's to switch to new infrastructure. Register a new domain, provision new mailboxes, warm them, resume.

This is why experienced operators always have a second set of domains ready. Infrastructure is disposable. Your offer, list, and copy are the durable assets.

What to do with this

• Verify every list against bounce-check services before sending, a 3%+ bounce rate trashes IP reputation fast
• Keep spam complaints under 0.1%, one complaint per 1,000 sends is the ceiling, above that reputation algorithms start throttling
• Maintain steady daily volume rather than spikes, erratic sending patterns look automated and trigger reputation dings
• Keep spare pre-warmed domains ready, when reputation damage is real, infrastructure replacement beats repair attempts
• Don't re-use burned domains hoping reputation will recover, once flagged, they rarely come back, retire and replace

Next: Content that triggers spam filters.