Definition
A sending IP address is the unique numerical identifier assigned to the mail server that transmits your email messages. Mailbox providers track the sending behaviour associated with each IP address, building an IP reputation score that influences whether messages from that IP are delivered to the inbox, routed to spam, or blocked entirely. Unlike domain reputation, which tracks the sender domain, IP reputation tracks the server infrastructure used to send the mail.
Dedicated IPs are assigned exclusively to one sender, giving you complete control over the reputation associated with that address. Shared IPs pool multiple senders together, meaning your deliverability is affected by the sending practices of every other user on that IP. Most email service providers (ESPs) assign new customers to shared IP pools initially, offering dedicated IPs only after the customer reaches a consistent sending volume of at least 50,000-100,000 emails per month.
The IP warmup process is the gradual increase in sending volume over 4-8 weeks that builds a positive reputation with mailbox providers. A cold IP sending large volumes suddenly is treated as suspicious behaviour and throttled or blocked. The warmup process trains receiving mail servers that the IP is legitimate by establishing a pattern of low complaint rates, low bounce rates, and high engagement.
Best Practices
Start on a shared IP pool if you send fewer than 50,000 emails per month. Shared pools benefit from the existing reputation of the pool, though you have less control. Only migrate to a dedicated IP once your sending patterns are consistent and your data hygiene is proven.
Warm up a dedicated IP over 4-8 weeks following a structured schedule. Begin with 200-500 emails per day to your most engaged subscribers in week one. Double volume weekly, never exceeding a 0.1% complaint rate or 3% bounce rate at any stage. Use Google Postmaster Tools to monitor IP reputation in real time during warmup.
Maintain separate IP pools for different email streams. Marketing campaigns, transactional messages, and operational notifications should each have their own IP or IP pool. This prevents a marketing campaign with high complaint rates from affecting transactional deliverability.
Monitor IP blacklisting across major blocklists daily. Use tools like MXToolbox, Spamhaus, and BarracudaCentral to check whether your IP appears on any blocklist. A listing on Spamhaus can take days to weeks to resolve and may require a formal delisting request.
If using IPv6, confirm that your ESP and all major mailbox providers support IPv6 connections from your sending infrastructure. As of 2024, Gmail, Outlook, and Yahoo all accept IPv6 connections, but not all ESPs support IPv6 outbound sending. Mixed IPv4/IPv6 setups must maintain consistent reputation across both protocols.
Frequently Asked Questions
Related Glossary Terms
Frequently Asked Questions
When you send consistently above 50,000 emails per month and have reliable data quality — low bounce rates (under 2%), low complaint rates (under 0.1%), and good engagement. The cost of a dedicated IP is justified when you need full control over reputation.
Most mailbox providers require 4-8 weeks of gradually increasing volume. Gmail is typically the most cautious provider during warmup. If your engagement rates are strong, you can usually reach full volume by week 6. Low engagement extends warmup to week 8 or longer.
Yes, and this is common for high-volume senders. IP rotation spreads volume across multiple addresses, reducing the per-IP sending load. Use round-robin or volume-based distribution, but ensure each IP goes through its own warmup process before receiving full volume.
Your deliverability drops sharply, often to zero for the affected mailbox providers. The resolution depends on the blocklist: some require a delisting request, others automatically delist after the problematic behaviour stops. Reduce volume, fix the root cause, and submit delisting requests promptly.
No, domain reputation is now generally weighted more heavily by Gmail and Yahoo, especially after the 2024 sender requirement updates. IP reputation still matters, particularly for new senders, but domain reputation is the primary factor for established senders.