Best Practices For Warming Mailboxes for Cold Outreach
Email warming up has been a standard practice in the cold email industry for years. The concept is simple: gradually increase your sending volume to build a positive reputation with email service providers before launching full-scale campaigns. But as we move through 2025, with increasingly sophisticated spam filters and AI-powered email security, many are questioning whether traditional warming practices still deliver results. This article examines the current state of email warming, its effectiveness, and whether it remains a necessary step in your cold email strategy.
What is Email Warming Up? 🌡️
Email warming up (also called inbox warming or email account warming) is the process of gradually establishing a positive sending reputation for a new email account or domain before sending cold email campaigns at scale. The process typically involves:
- Starting with low volume - Sending just a few emails per day initially
- Gradual increase - Slowly ramping up sending volume over time (usually 2-4 weeks)
- Positive engagement - Ensuring emails receive opens, replies, and other positive engagement signals
- Avoiding spam triggers - Carefully monitoring deliverability metrics to prevent reputation damage
The traditional warming approach has two main variants:
Manual warming - Sending emails to real contacts (colleagues, friends, or willing participants) who agree to open, reply to, and positively engage with your messages.
Automated warming - Using specialized tools that connect to networks of other accounts to automatically exchange emails, generate opens and replies, and rescue messages from spam folders.
The Traditional Warming Wisdom 📚
For years, the conventional advice for warming has been relatively consistent:
- Warm new domains and accounts for 2-4 weeks before sending cold emails
- Start with 5-10 emails per day and gradually increase by 5-10 daily
- Aim for high engagement rates (opens, replies, clicks)
- Avoid spam triggers like excessive links or attachments during warming
- Continue warming activities even after beginning cold outreach
This approach was based on the understanding that email providers like Gmail, Outlook, and Yahoo evaluate new senders cautiously and look for patterns of legitimate email behavior before granting good inbox placement.
The 2025 Email Landscape: What's Changed 🔄
The email deliverability landscape has evolved significantly in recent years, with several important developments affecting warming practices:
- AI-powered spam detection - Email providers now use sophisticated machine learning to identify patterns associated with cold email, regardless of warming
- Engagement-based filtering - Recipient engagement has become the dominant factor in deliverability, sometimes outweighing warming signals
- Domain reputation systems - More emphasis on domain-level reputation rather than just IP or account history
- Provider policy changes - Gmail and Microsoft have implemented stricter bulk sending requirements and authentication standards
- Automated warming skepticism - Email providers have become better at identifying and discounting artificial engagement from warming tools
Does Email Warming Still Work in 2025? 🧐
The answer is nuanced and depends on several factors:
Manual Warming: Still Valuable ✅
There's strong consensus among experts that manual warming with real, relevant contacts continues to provide benefits:
- Creates authentic engagement patterns that AI filters recognize as legitimate
- Establishes natural sending patterns that mimic regular business email use
- Builds genuine relationships that can provide ongoing engagement signals
- Allows for gradual volume increase that doesn't trigger suspicious activity flags
Nick Palasz, Email Deliverability Consultant, notes: "In 2025, manual warming with real contacts who genuinely engage with your content remains one of the most effective ways to establish sender reputation. This approach creates authentic engagement patterns that even the most sophisticated AI filters recognize as legitimate."
Automated Warming: Mixed Results at Best ⚠️
The effectiveness of automated warming tools has become increasingly questionable:
- Major email providers have improved detection of artificial engagement networks
- Some studies suggest automated warming may actually harm deliverability in certain cases
- The uniform patterns created by warming tools can appear suspicious to advanced filtering systems
- Results vary significantly between different automated warming services
A 2025 study by Email Deliverability Insights found that accounts using only automated warming tools saw an average inbox placement rate just 7% higher than completely unwarmed accounts—a much smaller benefit than reported in previous years.
Case Studies: Real-World Results 📊
Case Study 1: SaaS Company
A B2B SaaS company tested three identical domains with different warming approaches:
- Domain A: No warming (control)
- Domain B: 3 weeks of automated warming only
- Domain C: 3 weeks of manual warming with real contacts
Results after sending identical campaigns of 1,000 emails:
- Domain A: 62% inbox placement
- Domain B: 68% inbox placement
- Domain C: 86% inbox placement
The manually warmed domain significantly outperformed both the unwarmed domain and the automatically warmed domain.
Case Study 2: Marketing Agency
A marketing agency compared results from 20 client domains using automated warming tools in 2023 versus 20 new domains in 2025 using a hybrid approach (limited automated warming plus manual engagement):
- 2023 (automated only): Average 71% inbox placement
- 2025 (hybrid approach): Average 83% inbox placement
The hybrid approach showed significantly better results, suggesting that automated warming alone has become less effective.
Modern Warming Best Practices for 2025 🚀
Based on current evidence and expert consensus, here are the most effective warming practices for 2025:
- Prioritize manual warming - Engage with real contacts who will genuinely interact with your emails
- Focus on authentication - Properly implement SPF, DKIM, and DMARC before beginning any warming
- Warm the domain, not just the account - Establish domain reputation through various sending activities
- Be patient - Allow 3-4 weeks for proper warming, especially for new domains
- Start with transactional-style emails - Begin with one-to-one communications that resemble normal business email
- Monitor closely - Watch deliverability metrics and adjust if you see negative signals
- Consider hybrid approaches - Combine limited automated warming with manual engagement for best results
When Warming Matters Most 🔑
The importance of warming varies depending on your specific situation:
Warming is most critical when:
- Using a brand new domain (less than 30 days old)
- Starting with a new email service provider
- Recovering from deliverability issues or blacklisting
- Planning high-volume cold email campaigns (100+ emails/day)
- Targeting high-value prospects where inbox placement is crucial
Warming may be less important when:
- Using an established domain with good sending history
- Sending very low volumes (under 20 emails/day)
- Communicating primarily with contacts who have previously engaged
- Focusing on transactional rather than marketing emails
Conclusion: Evolution, Not Extinction 🌱
Email warming hasn't become obsolete in 2025, but its application has evolved. The data suggests that:
- Manual warming with real engagement remains valuable and effective
- Automated warming tools show diminishing returns as detection improves
- A holistic approach to sender reputation—combining proper authentication, quality content, and gradual volume increase—delivers the best results
Rather than abandoning warming practices entirely, successful email marketers in 2025 are adapting their approach to focus on authentic engagement and relationship building rather than artificial warming metrics.
The most important takeaway is that there are no shortcuts to good deliverability. While proper warming can help establish initial reputation, long-term inbox placement depends on sending relevant, valuable content to properly targeted recipients who genuinely want to engage with your messages.
In the evolving landscape of email deliverability, warming remains one tool in a comprehensive strategy—not a magic solution, but still a valuable step when implemented correctly.