I recently had the pleasure of moderating a CSA webinar on IP and domain warming, joined by two experts: Stéphane Decamps from Hornetsecurity and Laurent Garnier from Ikusita. The session provided hands-on experience and crucial insights into building trust and reputation for email senders and stood out for its practical perspective from the sender and the receiver/filter side of the ecosystem.

Instead of offering a one-way presentation, it became a valuable discussion about what really matters for trust-building when starting to send from new Ips or domains —what mailbox providers (MBPs) and filters look for, and how senders can align with those expectations from day one.

Stéphane highlighted that having no reputation is one of the riskiest positions for a sender, as mailbox providers are highly cautious of anything new. A significant part of new activity, he explained, turns out to be malicious – whether phishing attempts or senders trying to escape a poor reputation by switching IPs or domains. This underscores why warming up must be approached with care and strategy by every sender.

Why Warm-Up Is Non-Negotiable

Since the primary goal of MBPs and filters is always to protect end users from spam and fraud, new Ips or domains are scrutinized closely.

The core objective of warming up is to incrementally build trust with MBPs and filters by sending emails slowly and strategically. A proper warm-up helps senders progressively demonstrate to filters that their messages are appreciated and that they are legitimate.

Without this approach, emails are far more likely to be flagged as spam, experience high bounce rates, or be blocked outright.

How Reputation Is Built or Damaged

Sender reputation is like a credit score in the email ecosystem—it reflects sending behaviour and quality and is calculated based on:

• Bounce, Spam complaint rates and Spamtraps
• User engagement (opens, clicks, replies)
• List quality and List hygiene
• Sending consistency
• Authentication accuracy

During the warm-up period, the sender normally starts with no reputation or history – neither good nor bad – making every single action count heavily. Even a slight negative action, especially even some few spam reports, can have a large negative impact.

Key Technical Foundations fur a Trustworthy Setup

While many senders are familiar with email authentication, certain details are crucial for a smooth warm-up:

• SPF: List only IPs actually in use. Avoid generic “includes” that bring thousands of IPs into scope – making the task easier for cybercriminals.
• DKIM: Use strong (2048-bit) keys and rotate them every 6–12 months for higher security.
• DMARC: Start with p=none to monitor performance, then quickly move to quarantine or reject once authentication got proofed as set up correctly.
• rDNS: Ensure reverse DNS is configured correctly to reflect ownership of the IP.
• TLS: Secure transmission is now a standard expectation, especially with providers like Gmail.
• Dedicated IPs: shared IPs should be avoided to prevent inheriting poor reputation from other senders. Only dedicated IPs allow full control!
• Adjacent IPs: If using multiple IPs, choose ones that are close in range to avoid looking like a “snowshoe” setup. A wider range of disparate IPs can complicate the warm-up.

Building Trust Beyond Infrastructure

Warm-up isn’t just focusing on the technical aspects. Because there are, for example, compromised accounts, a filter cannot be based only on the trust in the sender’s IP/domain.

Many filters extract every element found in an email for decision making. Each of the found elements in the email can impact delivery and deliverability. This includes the sending network, other IPs in headers, domains used for images or URLs, email addresses, and even phone numbers or integrated advertisements.

To maintain maximum control, it’s essential to:

• Host images and URLs on the same domain as the used “from” address.
• Avoid unnecessary server hops that add unrelated IPs to headers – they can confuse filters and harm reputation.
• Work only with serious and trusted partners if using external advertising platforms for content – as their reputation will affect yours.
• Avoid using any element over which you lack reputation control.

MBPs, through their filtering systems, often also build reputation based on the combination of the “from” domain and the sending IP range. Using a small and consistent set of domains or sub-domains together with a stable IP setup helps build a clearer reputation signal and can reduce the risk of being incorrectly blocked.

Using too many IPs for low traffic can be a negative signal and make warm-up more difficult, as it may be seen as a “snowshoe” campaign. The reputation of both the main domain and any sub-domains will get considered, so a bad reputation for either will impact deliverability.

Building Trust with Recipients

Recipient engagement strengthens sender reputation, and smart branding can boost trust and performance.

BIMI: Laurent’s experience has shown that while BIMI is not directly required for warm-up, it supports inbox visibility and brand recognition. As it requires a strict DMARC policy (quarantine or reject) and can increase open rates by 10–21%, sending strong trust signals to MBPs.

Domain Strategy: Use a sub-domain of your main domain whenever possible – it likely has existing reputation and reinforces brand consistency. In contrast, a brand-new domain may look suspicious and lacks credibility with filters and recipients alike.

Strategy and Planning

Planning is where warm-up success begins. Many senders struggle to grasp that this analytical and preparatory phase is the most critical part of a successful warm-up.

Key factors to consider during planning:

• Timeline: A realistic warm-up takes at least 4 – 8 weeks – but it can take longer. Rushing due to deadlines, especially due to contract deadlines when switching ESPs, is highly harmful.
• Future Needs: Plan for the number of IPs, domains, or sub-domains you will need as you scale your sending volume.
• Audience: Map and segment on user engagement – Begin the warm-up with the most active, recently engaged users.
• Content: Make sure to have a content plan prepared that generate replies, clicks, and opens – high positive engagement from day 1 is the goal.
• Timing: Avoid peak times like Black Friday or the Holiday Period when MBPs, filters and recipients are overloaded, and you might be impacted by external factors.
• Teamwork: Ensure that everyone (the brand, the ESP, IT-Team, Marketing-Team, CRM-Team) is aligned and prepared, including content availability.

Warm-Up in Practice: Monitoring and Adaptation are everything

There is no “one-size-fits-all” or “magic formula” for warm-up. Success depends on real-time monitoring and quick adaptation to feedback from MBPs.

Real-time feedback is the sender’s best friend. SMTP logs are a “gold mine” of insight.

SMTP Logs: Your first line of defense—track bounces, delivery status, and 4xx errors.

Engagement:

• Open Rate: Below 10–15% may signal poor content or a stale list.
• Click Rate: Under 2% suggests weak relevance.
• Replies: Encourage them – positive replies build trust. (don’t use a no-reply-address)
• Spam Complaints: Stay under 0.1%.

How to Adapt:

• High bounces (>5%): Pause sending to that segment and clean the list.
• Low engagement: Test subject lines, personalize, and re-segment by activity.

Watch for transient errors (4xx codes)—they’re early warnings that the warm-up is hitting limits:

• 421: Too much, too fast
• 450: Greylisting – temporary delay
• 451: Server policy/rate limit hit
• 452: Too many recipients at once
• 429: Rate-limited due to volume or speed

Ignoring these signals tells MBPs the sender doesn’t care about sending quality. Email is a two-way street – listening to feedback is essential!

A Practical Warm-Up Plan

Warm-up plans must be tailored to each specific case – but here’s a simplified example to illustrate a possible approach:

Week 1: 100–500 emails/day to recent openers or better clickers

Week 2: If metrics are solid (bounce rate <5%, complaints <0.1%, open rates >15%), increase to 1,000/day

Weeks 3–8: Gradually scale to 10,000+/day, increasing by 50–100% per week as long as engagement remains strong and errors remain low

Important: This is only an example. Depending on the specific context—such as pre-existing reputation, audience quality, or infrastructure – warm-up may be completed more quickly or may require a longer, more cautious approach. Monitoring performance and adjusting in real time is always the key.

Never move to the next phase without seeing “green light” indicators on all major monitored metrics.

International Warm-Up & Troubleshooting

MBPs and filters around the world apply different thresholds and filtering logic – there is no global standard for sender reputation. They all have their own rules for volume, frequency, and acceptable behaviour, so warm-up must be adjusted accordingly. Again: Don’t use the one fits it all approach if you want to succeed!

Troubleshooting should always start with internal diagnostics and postmaster documentation. As Stéphane noted, announcing a warm-up to MBPs is usually unnecessary – they rely on automated systems, and they trust their systems. Only reach out if issues persist and you’ve clearly identified the root cause. Do your homework first.

Common Mistakes and Rebuilding Trust

A frequent mistake during warm-up is starting with volumes that are too high too quickly – often due to business pressure or misunderstanding how filter measure aggregate sending across multiple MBPs for example.

If trust is broken, it can be rebuilt – but doing so takes time and strategy. Rebuilding reputation is far harder than getting it right from the start. The key is not to stop sending altogether, but to reduce volume and focus on high-quality, engaged recipients.

A common myth is that switching IPs, domains, or ESPs will fix deliverability problems. As Stephane noted, “changing IPs or domains doesn’t change anything” if the root issue is poor sending behaviour. A new ESP only helps if it also supports better practices.

The Future: Brand Reputation & AI

Looking ahead, Stéphane and Laurent agreed that while IP and domain reputation are still essential, AI is shifting the landscape. MBPs and filter will increasingly evaluate brand-level behaviour and trust, not just technical identifiers. This means that brand credibility and consistency across messages will matter more than ever.

Final Takeaways

As our experts emphasized, warming-up is not just about hitting the send button – it’s about earning trust. Planning, Patience, Monitoring and agile Adaption with a focus on sending quality content to engaged users are paramount for building lasting trust and ensuring success resulting in stable deliverability.

Watch the full webinar on YouTube
Connect with our speakers on LinkedIn:

• Laurent Garnier – Ikusita
• Stéphane Decamps – Hornetsecurity
• Sandra Schubert – Certified Senders Alliance