Warmup Services Harm Your Deliverability
Blog post from Resend
Validity’s Heatwave is a blocklist designed to identify domains that appear to use synthetic email-warming services, scoring them through signals from its Intelligence Network and sharing the results with mailbox providers and security vendors such as Comcast, Proofpoint, Spamhaus, and SURBL. More than 1.2 million domains have reportedly been listed, and inclusion can harm sending reputation and inbox placement; the synthetic-warming designation is permanent once a domain reaches the threshold, although a separate cold-outreach classification may be removed when that activity stops. Organizations may be affected by warming performed by agencies, outbound vendors, or lookalike domains registered on their behalf, and can check their status through Validity’s lookup tool or request a review if they believe a listing is incorrect. The recommended alternative is to build reputation organically by authenticating domains, obtaining recipient consent, increasing sending volume gradually, and generating genuine engagement rather than relying on automated activity.
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