Mailwarm 2.0 (YC S20)

Amine Benjelloun and Gonzague Hacher built Mailwarm in Paris and got into Y Combinator's S20 batch—because when you're sending cold emails at scale, nothing kills your outreach faster than landing in the spam folder. They scaled to 10 employees, got acquired by Deliverability Experts in 2025, and their tech now powers mailX. That's a quiet exit, but the real story is what they built.

Email warmup sounds boring until you realize that 31% of your carefully crafted cold emails are disappearing into Microsoft's spam folder, or that your sending domain has zero reputation and every major inbox provider is treating you like a bot. Mailwarm fixes that by sending automated warm-up emails to real inboxes with positive engagement signals—building sender reputation over time so your actual campaigns actually land in the inbox. They claim 98% sending automation success, 88% reputation improvement, and 75% open rates across their network. Ten thousand teams use them, from scrappy startups to actual enterprises.

Here's the thing: everyone obsesses over email subject lines and copy, but if your sender reputation is trash, nobody's reading anything. Mailwarm is the unsexy infrastructure that makes your whole outreach stack actually work. They support Gmail, Outlook, Yahoo, Zoho, SendGrid, Mailgun—basically anywhere you're sending email from.Gonzague had founded two companies before this. Amine scaled the Paris team. They got acquired, which is the move. But inbox placement is still the biggest unsolved problem in cold outreach, and somebody's gotta own it.