What are best practices for warming up a new subdomain for email sending, and how does it impact DKIM alignment?

Key findings

• Gradual increase: The core of subdomain warming is a slow, steady increase in sending volume, often doubling email volume every few days, starting with low numbers (e.g., 1,000-5,000 emails/day).
• Audience selection: Start sending to your most engaged and active subscribers. This helps build positive engagement metrics from the outset.
• DKIM alignment: For DMARC, relaxed alignment means the DKIM domain (d=) and the From domain (header from) only need to share the same organizational domain, which is generally fine for subdomains.
• Strict DMARC policy: An edge case exists where a From domain might have a DMARC policy requiring strict alignment (exact match), though this is rare.

Key considerations

• Monitor performance: Keep a close eye on deliverability rates, bounce rates, and spam complaints. Adjust your sending volume based on these metrics.
• Content and audience impact: The warmup timeline heavily depends on your specific email content and the engagement level of your target audience.
• Pre-existing issues: Do not attempt to warm up a new subdomain if you have existing deliverability problems on your main domain. Resolve those first.
• Microsoft accommodations: While Microsoft's preemptive accommodations primarily apply to new IP addresses, submitting a ticket for a new subdomain may still be beneficial, even if the effect is minimal.

Key opinions

• Relaxed alignment is key: Many marketers agree that using a DKIM domain that is a subdomain of the From domain typically works well due to DMARC's relaxed alignment rules, which only require the organizational domain to match.
• Volume control: Starting with low volumes and gradually increasing them (e.g., doubling every few days) is a widely accepted best practice for warming up a subdomain.
• Engaged audience first: It's often recommended to send to your most engaged audience first to build a positive reputation signal with mailbox providers during the warm-up phase.
• Addressing underlying issues: Solving existing deliverability problems before attempting a subdomain warm-up is crucial to avoid transferring negative reputation.

Key considerations

• DMARC policy check: Always check if the From domain has a DMARC policy, especially if it's set to strict alignment, which could impact deliverability with a different DKIM subdomain.
• Content and list quality: The effectiveness of your warm-up is heavily influenced by the quality of your content and the responsiveness of your email list.
• ESP guidance: Marketers should consult their email service provider (ESP) for specific warm-up recommendations tailored to their platform and sending infrastructure. For migrating ESPs, this guidance is even more critical.
• Warm-up for existing vs. new audience: It's generally better to warm up a new subdomain with an existing, highly engaged audience before introducing it to newer or less engaged contacts.

Key opinions

• Pre-warming with old mail: An effective strategy for warming a new subdomain involves signing existing (old) mail with the new subdomain in a new DKIM signature for a period. This introduces the new subdomain and associates it with a known, good mail stream.
• Domain reputation transfer: Subdomains inherit some reputation from the root domain, but they still require their own warm-up to establish specific sending patterns and build trust for their unique mail streams.
• Consistency is key: Consistent sending volumes and content during the warm-up period help mailbox providers accurately assess sender behavior and assign a positive reputation.
• Engagement feedback: Actively seeking and responding to positive engagement signals (opens, clicks) and negative ones (complaints, bounces) is vital for adjusting warm-up strategy.

Key considerations

• DMARC policy review: Prior to warming, verify the DMARC policy of the root domain. A strict policy (`p=reject` or `p=quarantine`) could cause issues if the DKIM subdomain isn't perfectly aligned.
• Monitoring tools: Utilize postmaster tools (e.g., Google Postmaster Tools) and blocklist checkers to actively monitor the new subdomain's reputation.
• DNS configuration: Ensure all necessary DNS records, including SPF and DKIM, are correctly configured for the new subdomain before beginning any sending.
• Segmenting traffic: For large senders, gradually shifting a small percentage of overall email traffic to the new subdomain can minimize risk during the warm-up.

Key findings

• Authentication importance: Documentation consistently stresses the fundamental requirement of properly configured SPF, DKIM, and DMARC records for any sending domain or subdomain.
• DMARC alignment rules: Official guides explain that DMARC's relaxed alignment for DKIM allows a subdomain's DKIM signature to align with the From header's organizational domain, generally making subdomain warming compatible with DMARC.
• Gradual volume escalation: Many best practice documents recommend a slow, controlled increase in sending volume, often with predefined daily or weekly increments, to allow ISPs to assess the sender's behavior without triggering spam filters.
• Engagement feedback loops: Mailbox providers offer feedback loops (FBLs) that allow senders to receive notifications when recipients mark their emails as spam, which is crucial for adjusting warm-up strategy.

Key considerations

• Dedicated vs. shared IPs: The warm-up process might differ slightly depending on whether the subdomain is associated with a dedicated IP or a shared IP pool.
• Content quality and consistency: High-quality, desired content sent consistently helps build a positive sender reputation for the new subdomain more effectively.
• Segmentation during warm-up: Targeting the most active and engaged segments of your audience first is a common recommendation to ensure positive initial interactions.
• Monitoring tools integration: Leveraging Postmaster Tools and DMARC reports is essential for tracking subdomain reputation and identifying any deliverability issues during the warm-up period.