Cold Email Infrastructure: Google Workspace vs Microsoft 365 vs SMTP Relays

Most outbound campaigns fail before a single subject line is written. The sending setup underneath them — the mailboxes, domains and routing that actually deliver the message — is chosen on price or habit, then blamed later when replies dry up. Your cold email infrastructure decides how many emails you can send safely, how fast a mistake burns you, and how much of your list ever reaches an inbox. Here is how the three realistic options compare, and how to choose between them.

What cold email infrastructure actually includes

The phrase gets used loosely. In practice it is four layers stacked together:

• Domains — your brand domain plus the lookalike domains you send from. If you have not separated these yet, start with our guide to secondary domains for cold outreach.
• Mailboxes — the individual inboxes attached to each domain, usually two or three per domain.
• Authentication — SPF, DKIM and DMARC records that prove you are allowed to send. See our email authentication setup guide.
• Routing — the mail service that actually hands your message to the recipient server.

The first three layers look nearly identical whichever provider you pick. Routing is the real decision.

Option 1: Google Workspace

Google Workspace is the default choice for most outbound teams, and for good reason. Gmail-to-Gmail delivery is strong, the admin console is simple, and almost every sending tool integrates with it natively.

• Cost: roughly $7–$14 per mailbox per month depending on tier. This is the main constraint — a 30-mailbox setup is a real line item.
• Practical send limit: Google allows far more on paper, but safe cold sending is 30–50 emails per mailbox per day. Our breakdown of cold email sending limits explains why the safe number is so much lower than the published one.
• Strengths: excellent reputation signals, easy DKIM setup, reliable API access for sending tools, strong spam filtering on inbound replies.
• Weaknesses: Google is aggressive about suspending accounts it thinks are bulk-sending. Bulk-created mailboxes on a new domain get flagged quickly.

Choose Workspace when your prospects are predominantly on Gmail, when you want the lowest-friction setup, and when the per-seat cost fits your volume.

Option 2: Microsoft 365

Microsoft is the quieter option and frequently the smarter one for enterprise-facing outbound.

• Cost: around $6 per mailbox per month on Business Basic, and Microsoft permits multiple aliases per licensed mailbox — which materially lowers cost per sending identity.
• Practical send limit: similar territory, 30–50 per mailbox per day for cold traffic.
• Strengths: if your ICP is mid-market or enterprise, most of those inboxes are Outlook. Sending Microsoft-to-Microsoft avoids one layer of cross-provider suspicion. Alias support is the underrated advantage.
• Weaknesses: the admin experience is heavier, DNS setup takes longer, and Microsoft Defender can be unforgiving about link-heavy or attachment-bearing messages.

Choose Microsoft when you sell into companies over roughly 200 employees, or when you want more sending identities per dollar.

Option 3: SMTP relays and private mail servers

The third path is sending through a dedicated SMTP service or your own mail server rather than a mailbox provider. Costs drop sharply and volume ceilings disappear — but so does the safety net.

• Cost: often a few cents per thousand emails, or a flat monthly fee for a dedicated IP. Dramatically cheaper at volume.
• Strengths: no per-seat pricing, no arbitrary provider suspensions, full control over IP reputation, and the ability to send from unlimited addresses on a domain.
• Weaknesses: you own the reputation entirely. A fresh IP has no history, so you must warm it slowly. Shared-IP relays can put you behind someone else’s bad behaviour. Many relay providers also prohibit cold outreach in their terms of service — read them before you commit.
• Reply handling: relays send but do not always receive well. You still need real mailboxes for the conversations that follow.

Choose a relay only when you are sending high volume, have someone who understands IP warming, and have accepted that reputation management is now your job.

A simple decision path

1. Under 5,000 emails a month? Google Workspace or Microsoft 365. Do not overbuild.
2. Selling to enterprise? Microsoft 365, for inbox-provider alignment and alias economics.
3. Selling to startups, agencies and SMBs? Google Workspace.
4. Above 50,000 emails a month with in-house technical support? Evaluate a dedicated relay, but keep mailbox-based sending for your highest-value segments.
5. Unsure? Split the difference: run mailboxes for tier-one accounts and a cheaper channel for broad, low-stakes campaigns.

Building the stack, step by step

1. Register secondary domains. Two to four close variants of your brand domain. Never send cold volume from your primary.
2. Point DNS and set up authentication. SPF, DKIM and a DMARC policy starting at p=none while you monitor reports.
3. Create two to three mailboxes per domain. Real names, real job titles, real profile photos. Empty-looking senders get filtered.
4. Warm every mailbox for two to three weeks before it touches a prospect. Our email warmup guide covers the ramp schedule.
5. Cap daily sends at 30–50 per mailbox and increase only if bounce and spam-complaint rates stay clean.
6. Monitor placement monthly, not just opens. Where mail lands is a separate question from whether it was delivered.

Mistakes that burn infrastructure fastest

• Skipping warmup. A brand-new mailbox blasting 100 emails on day one is the single most common cause of a burned domain.
• One domain, many mailboxes. Concentrating all sending on a single domain means one bad campaign takes down everything.
• Mixing cold and transactional sending. Keep your invoicing, product and support mail entirely off cold-sending domains.
• Ignoring bounces. A dirty list will destroy the best infrastructure in a week. Verify before you send.
• Chasing cheap seats. Saving $4 a mailbox is meaningless if the setup costs you a quarter of pipeline.

The bottom line

There is no universally correct provider. Google Workspace is the safe default for SMB-facing outbound, Microsoft 365 wins on enterprise alignment and alias economics, and SMTP relays only make sense at volume with technical ownership. Whichever you choose, the infrastructure is only as good as the discipline around it: separate domains, authenticated records, warmed mailboxes, conservative caps and a clean list. Get those right and the provider debate stops mattering much. For the wider picture, see our guide to email deliverability.