Teams turn on a dedicated IP as soon as budget allows, on the assumption that an address tied to the brand earns more trust from mailbox providers. Below a specific volume threshold, the opposite happens: a brand-new IP with no history delivers worse than a shared IP that thousands of other senders have already built a reputation on. The number to remember sits around 100,000 emails a month as a practical floor, with a comfortable zone between 250,000 and 1 million. Below that, pooled reputation protects you. Above it, a dedicated IP starts to make sense, provided it gets warmed up over several weeks before reaching target volume.
The shared IP pool: a default that holds up
A small business sending 8,000 newsletters a month shares its IP address with hundreds of other senders through its provider. It has never had to think about it: inside a shared IP pool, deliverability inherits a reputation already built by the combined volume of every sender in the pool. The quality of that pool depends entirely on how well the provider maintains it, in principle weeding out bad senders. A careless neighbor can, in theory, drag down everyone else’s score. Serious ESPs watch this risk closely.
The volume threshold that tips the scale toward a dedicated IP
That number is exactly what most general-purpose guides leave vague. Mailgun puts the average dedicated IP threshold at one million emails sent per month, while deliverability scales published in 2026 set a practical floor at 100,000 monthly emails, with a comfortable zone between 250,000 and 1 million, enough signal to absorb an incident without losing everything. Below 20,000 emails a month, none of these sources recommend leaving the shared pool: volume alone can’t build a history that Gmail, Outlook or Yahoo can read. The threshold guarantees nothing by itself. A high volume mostly buys room to recover after an incident, not automatic reputation: a poorly segmented dedicated IP sending 500,000 emails a month degrades just as fast as a badly managed small shared IP.
Reverse DNS, the box everyone forgets to tick
Does a missing PTR record block a send today? Yes. That wasn’t true three years ago. Reverse DNS ties an IP address to a hostname, and it’s the first thing the receiving server checks, before the SMTP EHLO exchange even starts, at connection time. Google and Yahoo made PTR mandatory for senders of more than 5,000 emails a day in February 2024. Microsoft followed in May 2025 with a permanent 550 5.7.15 rejection for non-compliant IPs on Outlook, Hotmail and Live. Gmail moved from a simple deferral to an outright rejection in November 2025. On a brand-new dedicated IP, forgetting the PTR is still the most common mistake: hosts and ESPs almost never set this field by default, it’s on the sender to ask for it. On a shared IP, the question never comes up, the PTR is already in place for the whole pool.
Warming up a dedicated IP, week by week
An IP warm-up schedule isn’t a three-day formality. Based on the daily thresholds documented in 2026, a realistic ramp toward 500,000 emails a day looks like this.
- Week 1: up to 1,000 emails a day, limited to contacts who opened or clicked in the last 30 days.
- Week 2: up to 5,000 emails a day, still on the most engaged audience.
- Week 3: up to 15,000 emails a day, gradually widening to contacts active over 60 days.
- Week 4: up to 35,000 emails a day, with daily monitoring of the complaint rate.
- Week 5: up to 75,000 emails a day.
- Week 6: up to 150,000 emails a day, the point where early deferral signals need attention if they start climbing.
Mailjet recommends, in its documentation, reserving the first two weeks for contacts who opened or clicked recently: they respond better and report an email as spam less often, which protects the score at the most fragile point of the ramp-up. Past week 6, the ramp continues in 20-30% steps as long as the signals stay clean: few deferrals, few complaints, few hard bounces, stable delivery times. Slowing down beats a forced plateau. The full protocol, including for a fresh shared IP, is covered in the IP and domain warm-up guide.
Keeping marketing traffic separate from transactional traffic
A transactional order-confirmation email has nothing in common, reputation-wise, with a promotional newsletter sent to 200,000 contacts. The two flows react differently to complaints and open rates. Mixing them on the same IP creates a simple risk: a poorly targeted marketing campaign that drags down the score also delays the invoice or the password-reset link sent through transactional. Most ESPs offer a separate dedicated IP for each flow past a certain volume. Amazon SES, for instance, natively separates its dedicated pools from the default shared pool, an option frequently cited in high-volume ESP migrations.
Switching ESP without losing your reputation
IP reputation stays tied to the address itself, never to the brand using it or the contract in place, so changing providers resets that counter to zero. A company moving 300,000 emails a month from one ESP to another starts over with a brand-new address. Whether that address is dedicated or shared depends on the plan chosen. A full warm-up becomes necessary again if it opts for dedicated. Amazon SES manages its dedicated managed IPs with a gradual traffic split between the old and new address during the transition, an approach that limits the sharp deliverability drop at cutover. The number to watch during migration remains sender score, far more than the raw number of emails sent.
What takes an IP down: spam traps, blacklists, feedback loops
A 30% drop in deliverability over three months, with no change in content or sending frequency, signals IP reputation drift rather than a creative problem, often invisible as long as the sending tool keeps reporting a normal delivery status, the typical case where campaigns land in spam with no alert on the platform side.
Spam traps account for a large share of these drops: they are addresses created or reactivated by mailbox providers to catch poorly maintained lists, never used to receive real conversations. A single spam trap hit repeatedly is enough to push an IP onto a blacklist. The blacklist check and removal guide covers the procedure step by step. A hard bounce has a similar effect once it accumulates: every address that no longer exists but keeps getting emailed feeds a hard-bounce rate that damages the sender score before any alert shows up in the marketing tool. Catch-all addresses never trigger an immediate hard bounce, which delays detecting the drift even further.
The feedback loop (FBL) offered by Gmail, Outlook and Yahoo acts as a separate warning signal from bounces: it reports, in near real time, complaints from recipients who click “report as spam” instead of unsubscribing. Registering for the feedback loop removes a complaining contact before it weighs on the overall complaint rate.
A complaint rate above 0.3% is enough to push Gmail into outright rejection for bulk senders, according to the bulk sender rules Google has published since February 2024.
Filtering after the fact, once the campaign has already gone out, amounts to treating a symptom once the reputation is already damaged. Before raising volume on a fresh IP or switching addresses, regular list hygiene stays the cheapest move available: one avoided hard bounce is worth more, over time, than one extra email sent.
What a dedicated IP costs
Dedicated IP pricing varies widely by ESP and subscription tier. Some ESPs charge an extra 30 dollars a month for each additional dedicated IP. Other providers bundle it in from a certain monthly volume tier. Below that tier, they bill it separately. The math doesn’t stop at the sticker price: a dedicated IP running below the volume threshold covered above costs more in lost deliverability than in monthly subscription fees.
Configuration mistakes that undo everything else
The published SPF record still points to the old email provider three years after the contract ended: this is the case that shows up most often in deliverability audits, well ahead of a missing PTR. The alignment between SPF, DKIM and DMARC needs rechecking every time the IP address changes, regardless of its type: an SPF record that doesn’t include the new address breaks authentication, even when DKIM signs every message correctly.
Google Postmaster Tools and Microsoft SNDS (Smart Network Data Services) remain the two free dashboards for tracking IP reputation in near real time on the Gmail and Outlook side. A visible dip on either one, often before the open rate drops far enough for marketing leadership to ask questions, should trigger a list check rather than another campaign.
An IP address is only worth the history it’s allowed to build: warmed up too fast or neglected too long, it goes back to being an anonymous number in a DNS record, whatever budget went into dedicating it.
