Cold Email Infrastructure Types Explained: GWS vs MS365 vs Azure vs SMTP

By Puzzle Inbox Team · May 4, 2026 · 11 min read

Cold email providers fall into 4 infrastructure categories with different deliverability profiles. Here is the complete guide for choosing the right type.

The Four Cold Email Infrastructure Categories

Cold email providers can be grouped into four infrastructure categories based on underlying technology: Real Google Workspace, Real Microsoft 365, Azure/Entra tenant-based, and Private SMTP. Each has distinct deliverability profiles, pricing, and use cases. Understanding the categories helps you choose the right provider for your cold email operation.

Category 1: Real Google Workspace (Authentic GWS Accounts)

What it is: Genuine Google Workspace accounts provisioned through Google Cloud partnerships or direct resellers. Send through Google's IP infrastructure.

Deliverability advantage: Inherits Google's established IP reputation. Best deliverability to Gmail recipients.

Providers: Puzzle Inbox, Primeforge, InboxKit, Mailpool, Premium Inboxes, Maildoso (Combo plans), plus ~20 others offering authentic GWS.

Pricing: $2-10/inbox typical range.

Best for: Most cold email operations, especially Gmail-heavy prospect bases.

Category 2: Real Microsoft 365 (Authentic MS365 Accounts)

What it is: Genuine Microsoft 365 tenant accounts. Access to admin.microsoft.com. Send through Microsoft's standard email infrastructure.

Deliverability advantage: Best deliverability to Outlook/Hotmail/Live recipients. Microsoft-to-Microsoft trust.

Providers: Puzzle Inbox Outlook, Primeforge, InboxKit, Mailpool, Premium Inboxes, HyperInboxes, plus 8+ others.

Pricing: $0.35-5/inbox range.

Best for: Microsoft-heavy prospect bases or platform diversification alongside GWS.

Category 3: Azure/Entra Tenant-Based

What it is: Microsoft cloud infrastructure with dedicated tenants, but not standard MS365 configuration. Higher mailbox density possible.

Deliverability profile: Inherits Microsoft cloud reputation but with different sending patterns than MS365.

Providers: Hypertide, Sending.ac, InboxKit Enterprise, Inframail.

Pricing: Often unlimited-tenant pricing models.

Best for: High-volume Microsoft-heavy cold email with 50+ mailboxes per domain.

Category 4: Private SMTP / Custom Infrastructure

What it is: Custom mail servers and dedicated IPs operated by the provider. Not GWS, not MS365, not Microsoft cloud.

Deliverability profile: Zero starting reputation, requires extensive warmup. Best at high volume where dedicated IP reputation builds over time.

Providers: Mailreef, Infraforge, Aerosend, Mailforge, Superwave, SkySenders, plus 20+ others.

Pricing: Wide range from $0.40/inbox (shared SMTP) to $5,000+/month (enterprise dedicated IP).

Best for: 100K+ emails/day operations with deliverability engineering teams.

How to Choose the Right Category

By Volume

Under 5,000/day: Real Google Workspace wins
5,000-50,000/day: Mix of Real GWS + Real MS365 for diversification
50,000-100,000/day: Add Azure/Entra for density
100,000+/day: Consider dedicated IP SMTP for reputation isolation

By ICP

Gmail-heavy: Real GWS primary
Outlook-heavy: Real MS365 primary
Mixed: Both GWS and MS365 for diversification

By Technical Resources

No engineering: Real GWS or Real MS365 only
Some engineering: Azure/Entra works
Dedicated deliverability team: SMTP becomes viable

Mixing Infrastructure Categories

Serious cold email operations often mix categories:

70% Real Google Workspace (primary deliverability)
20% Real Microsoft 365 (Outlook diversification)
10% Azure/Entra or SMTP (volume backup)

This mix provides reputation diversification and platform-specific deliverability advantages.

Real Google Workspace and Real Microsoft 365 are the default choices for 90% of cold email operations. Azure/Entra and private SMTP are specialist choices for specific high-volume or technical use cases. Puzzle Inbox provides pre-warmed Real GWS + Real MS365 from one provider.