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, 99% inbox guarantee, and 2-4 hour setup. Minimum order: 50 mailboxes.
Sending.ac
Sending.ac is an Azure-based cold email infrastructure provider offering advanced infrastructure at aggressive pricing. Current pricing is $0.40/mailbox (increasing to $0.44 in April 2026, with plans to reach $1.00+). Minimum order is 600 mailboxes (~1,200 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 |
|---|---|---|
| Current Price per Mailbox | $1/mailbox (1-199), scaling to $0.55 at 5K+ | ✓ $0.40/mailbox (increasing to $0.44, then $0.50, then $1.00+ planned) |
| Long-term Pricing Stability | ✓ Stable published pricing — no announced increases | Active price increase roadmap: $0.40 → $0.44 → $0.50 → $1.00+ over coming months |
| Minimum Order | ✓ 50 mailboxes ($50/month) — accessible entry point | 600 mailboxes ($240/month) — 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 | 2-4 hours 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 | ✓ 99% 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. 600 mailboxes) and pricing stability. Sending.ac's $0.40/mailbox is the cheapest in the market right now, but they've publicly announced a roadmap to $1.00+ per mailbox — meaning the price gap will close or reverse. ColdRelay's 99% 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 600-mailbox minimum ($240/month). ColdRelay's pricing is also stable — you won't need to lock in early to avoid increases.
Where Sending.ac Wins
Sending.ac's current $0.40/mailbox pricing is the most aggressive in the cold email infrastructure space — less than half of ColdRelay's base rate. For large-volume operations already committed to 600+ mailboxes, the savings are significant: 1,000 mailboxes on Sending.ac costs $400/month vs. $700 on ColdRelay. The grandfathering policy means early adopters lock in current pricing as rates increase. 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 600 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 600+ mailboxes and want the lowest possible per-mailbox price right now. Lock in the current $0.40 rate before it increases. 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 currently offers the lowest per-mailbox pricing in the market, making it the cost leader for large-volume operations (600+ mailboxes). However, their publicly announced price increases will eventually bring pricing close to ColdRelay's. ColdRelay wins on accessibility (50 vs. 600 minimum), pricing stability, and its 99% inbox guarantee. If you need 600+ mailboxes today and want the cheapest rate, Sending.ac is hard to beat. If you want flexibility, guarantees, and long-term pricing certainty, ColdRelay is the safer bet.
Frequently Asked Questions
Why is Sending.ac so cheap right now?
Sending.ac has stated they're intentionally underpriced to build market share, with a published roadmap to increase prices from $0.40 to eventually $1.00+ per mailbox. Early customers are grandfathered at their signup price.
What's the minimum order for each platform?
ColdRelay: 50 mailboxes ($50/month). Sending.ac: 600 mailboxes (~$240/month). ColdRelay has a much lower barrier to entry.
Will Sending.ac's pricing eventually match ColdRelay's?
Based on their published 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 99% 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.