Overview — What Each Platform Does
ColdRelay
ColdRelay provisions Microsoft 365 mailboxes on dedicated, isolated Azure tenants with dedicated IPs. Per-mailbox pricing: $1 (1-199), $0.85 (200-999), $0.70 (1K-4,999), $0.55 (5K+). Includes 100-150 mailboxes per domain, auto DNS, no warmup wait, 95% inbox guarantee, and 60-minute setup. Minimum order: 50 mailboxes.
Sending.ac
Sending.ac is an Azure-based cold email infrastructure provider with an aggressive low-price-now, raise-prices-later strategy. As of mid-2026 the per-mailbox price is $0.44 (stepped up from an initial $0.40 in April 2026), with a further increase to $0.50 imminent and a standard rate of $1.00; verify the current rate on their site before relying on a specific figure. Minimum order is 450 mailboxes (~1,800 outreach emails/day). Sending.ac emphasizes fully isolated infrastructure, replaceable mailboxes, and 1-click setup in 15 seconds. They offer both aggressive and conservative setup modes, with API auto-import to sending tools. Active customers are grandfathered at their signup price.
Feature Comparison
| Feature | ColdRelay | Sending.ac |
|---|---|---|
| Published Price Trajectory | ✓ $1/mailbox (1-199), scaling to $0.55 at 5K+ — stable | Originally $0.40/mailbox; stepped up to $0.44 in April 2026 (current rate), with $0.50 imminent and a $1.00 standard rate. Verify current rate on sending.ac |
| Long-term Pricing Stability | ✓ Stable published pricing — no announced increases | Active price-increase roadmap: $0.40 → $0.44 → $0.50 → $1.00+ — early-customer grandfathering offered |
| Minimum Order | ✓ 50 mailboxes ($50/month) — accessible entry point | 450 mailboxes — higher minimum commitment |
| Infrastructure Isolation | Isolated Azure tenants with dedicated IPs | Fully isolated infrastructure per customer — replaceable mailboxes |
| DNS Configuration | Automated SPF, DKIM, DMARC | Automated DNS setup included |
| Setup Speed | 60 minutes for full deployment | 1-click setup in 15 seconds (provisioning), created in hours |
| Setup Flexibility | Standard automated setup | ✓ Choice between aggressive and conservative setup modes with customization in 2 minutes |
| API Integration | Compatible with Instantly, Smartlead, and other platforms | ✓ Auto-import via API to sending tools — connects API keys for automatic mailbox import |
| Warmup | No warmup required — send from day 1 | Infrastructure optimized for immediate use |
| Inbox Guarantee | ✓ 95% inbox placement guarantee with money-back promise | No published inbox guarantee |
| Grandfathering | Stable pricing — no grandfathering needed | Active customers grandfathered at their signup price as prices increase |
| Mailboxes per Domain | ✓ 100-150 per domain | Configuration varies based on aggressive vs. conservative setup choice |
Where ColdRelay Wins
ColdRelay's biggest advantages over Sending.ac are the low minimum order (50 vs. 450 mailboxes) and pricing stability. Sending.ac launched as the cheapest provider in the market, but they've publicly announced a price-increase roadmap heading toward $1.00+ per mailbox — meaning whatever gap exists today will keep closing. ColdRelay's 95% inbox guarantee with a money-back promise gives quantifiable deliverability assurance that Sending.ac doesn't offer. For teams starting small or testing the waters, ColdRelay's 50-mailbox minimum ($50/month) is far more accessible than Sending.ac's 450-mailbox minimum. ColdRelay's pricing is also stable — you won't need to lock in early to avoid increases.
Where Sending.ac Wins
Sending.ac's per-mailbox pricing has historically been among the most aggressive in the cold email infrastructure space — even after the published step-ups, it remains below ColdRelay's base rate for now (verify the current price on their site before committing). For large-volume operations already committed to 450+ mailboxes, the savings can be meaningful. The grandfathering policy means early adopters lock in their signup rate as prices rise. Sending.ac's setup flexibility (aggressive vs. conservative modes) and auto-API import to sending tools show thoughtful product design. They also survived a wave of Azure provider shutdowns, which suggests robust infrastructure.
✓ Choose ColdRelay If…
Choose ColdRelay if you want pricing stability, a low entry point, and the confidence of an inbox placement guarantee. ColdRelay is the better fit for teams starting with fewer than 450 mailboxes, operators who value predictable long-term costs, and anyone who wants a concrete deliverability guarantee backing their infrastructure.
Choose Sending.ac If…
Choose Sending.ac if you're committed to 450+ mailboxes and want to lock in their current published rate before the next step-up. Sending.ac is ideal for large-scale operations that prioritize raw cost per mailbox and are comfortable with a higher minimum order and evolving pricing.
The Verdict
Sending.ac has historically offered the lowest per-mailbox pricing in the market, making it the cost leader for large-volume operations (450+ mailboxes). However, their publicly announced price-increase roadmap will eventually bring pricing close to ColdRelay's. ColdRelay wins on accessibility (50 vs. 450 minimum), pricing stability, and its 95% inbox guarantee. If you need 450+ mailboxes today and want to lock in Sending.ac's current rate before the next step-up, they're hard to beat. If you want flexibility, guarantees, and long-term pricing certainty, ColdRelay is the safer bet.
Frequently Asked Questions
Why was Sending.ac so cheap?
Sending.ac launched intentionally underpriced to build market share, with a public roadmap to step prices up over time — from an initial $0.40/mailbox, it stepped up to $0.44 in April 2026 (the current rate) and is heading toward $0.50 and a $1.00 standard rate. Early customers are grandfathered at their signup price. Verify the current published rate on sending.ac before relying on a specific figure.
What's the minimum order for each platform?
ColdRelay: 50 mailboxes ($50/month). Sending.ac: 450 mailboxes. ColdRelay has a much lower barrier to entry.
Will Sending.ac's pricing eventually match ColdRelay's?
Based on their published step-up roadmap ($0.40 → $0.44 → $0.50 → $1.00+), yes. Sending.ac plans to reach $1.00+ per mailbox, which is ColdRelay's current base rate. Existing Sending.ac customers keep their grandfathered rate.
Does Sending.ac offer an inbox placement guarantee?
No. ColdRelay offers a 95% inbox placement guarantee with a money-back promise. Sending.ac doesn't publish a specific deliverability guarantee.
Both use Azure infrastructure — what's the actual difference?
Both run on Azure/Microsoft 365, but ColdRelay emphasizes isolated tenants, dedicated IPs, and a deliverability guarantee. Sending.ac emphasizes fully isolated infrastructure with replaceable mailboxes and auto-API integration. The core technology is similar; the differences are in pricing, minimum orders, and guarantees.