Blog / Infrastructure

Microsoft 365 vs. Google Workspace vs. SMTP for Cold Email

13 June 2026

If you send cold email, the inbox you send from matters as much as the message. The platform behind that inbox shapes how you set things up, how isolated you are from other senders, how your reputation is built, and what it costs. Three options come up repeatedly: Microsoft 365 mailboxes, Google Workspace mailboxes, and raw SMTP services. None is “the right answer” for everyone. This post compares them across the dimensions that actually affect outbound, and ends with the trade-offs rather than a verdict.

First, two definitions. A mailbox is a real email account (you@yourdomain.com) hosted by a provider, with its own inbox, sent folder, and sending reputation. SMTP (Simple Mail Transfer Protocol) is the underlying standard for sending mail; an “SMTP service” here means a relay or API you push messages through, often with no real inbox attached.

Setup

Setup covers how hard it is to get from a bought domain to a sending mailbox that your cold-email tool can use.

Microsoft 365. You provision a tenant (an isolated Microsoft 365 environment), add your domain, create mailboxes, and publish DNS records (SPF, DKIM, DMARC). Done by hand this is several steps per domain, and at scale it is repetitive. The upside is that each mailbox is a standard M365 inbox that connects to common cold-email sequencers.

Google Workspace. Similar shape: create a Workspace account, verify the domain, add users, set DNS. Google’s admin console is approachable, and DKIM is straightforward to turn on. The friction tends to show up later, in policy and verification, more than in initial setup.

Raw SMTP. Often the fastest to start sending: sign up, get credentials or an API key, point your tool at the relay. But you still own all the deliverability scaffolding yourself, and many SMTP relays are built for transactional or bulk mail, not for warming and rotating individual cold-outreach mailboxes.

Isolation

Isolation describes how separated your sending is from other senders. It matters because a neighbour’s bad behaviour can drag down a reputation you share.

Microsoft 365. A tenant is a real, isolated environment. Your mailboxes live in your own tenant rather than a shared pool, which is a meaningful difference from shared-IP mailbox products. Outbound flows over Microsoft’s infrastructure, so you are not the only sender on that infrastructure, but your tenant and domain identity are yours.

Google Workspace. Comparable in principle: a Workspace account with your own domain and users. Reputation attaches largely to your domain and sending behaviour rather than to a pool you share with strangers.

Raw SMTP. This is where isolation varies the most. Some relays put many customers on shared IP ranges, so reputation is partly communal. Dedicated IPs exist but cost more and must be warmed carefully. Read the specific service’s model before assuming you are isolated.

Reputation model

Reputation is how mailbox providers (the systems receiving your mail) decide whether to inbox, spam-folder, or reject you. Cold email is unsolicited by definition, so this is the dimension that breaks most outbound programs.

For both Microsoft 365 and Google Workspace, reputation is tied to your domain and the behaviour of your mailboxes: authentication (SPF, DKIM, DMARC), volume, ramp-up speed, bounce rates, spam complaints, and engagement. Sustained cold-email volume wears a domain down over time. Domains “burn” — their reputation degrades to the point where mail lands in spam — often within a few months, and faster at higher daily volume per mailbox. Treat that timing as approximate, not a guarantee. The common, durable pattern is to run several overlapping domains at modest per-mailbox volume and rotate them, rather than pushing one domain hard.

For raw SMTP, reputation depends heavily on the relay’s design and your IP situation. On shared IPs you inherit some of the pool’s standing; on dedicated IPs you carry your own. Either way, the same fundamentals apply: authenticate properly, ramp slowly, keep complaints low.

A note that applies to all three: no infrastructure choice makes cold email legally permissible. Where you host or which IPs you send from is about deliverability and data location, not consent. Germany’s UWG §7(2) and many EU markets require opt-in even for B2B. The sender is always responsible for consent and local law.

Cost structure

Cost is not just the sticker price; it is how the price scales as you add domains and mailboxes.

Microsoft 365 and Google Workspace are usually priced per user (per mailbox) per month, in tiers. Running cold outbound means many mailboxes across several domains, so cost grows with mailbox count. Add the time to provision, authenticate, monitor, and rotate, and the real cost is licences plus labour.

Raw SMTP is often priced by volume — emails or API calls per month — which can look cheap at low volume. But for cold outreach you typically still need real mailboxes for replies and credibility, so a pure SMTP relay rarely stands alone.

Side-by-side

Dimension	Microsoft 365	Google Workspace	Raw SMTP
Setup effort	Moderate; tenant + DNS per domain	Moderate; account + DNS per domain	Low to start; you own deliverability
Isolation	Real, isolated tenant	Own domain/account	Varies; often shared IPs
Reputation tied to	Domain + mailbox behaviour	Domain + mailbox behaviour	Relay design + IP standing
Real inbox for replies	Yes	Yes	Often no
Cost model	Per mailbox / month	Per mailbox / month	Per volume (often)
Cold-outbound fit	Strong, with rotation	Strong, policy-sensitive	Niche; rarely standalone

So which one?

The honest answer is that it depends on what you value.

Choose raw SMTP if you want the lowest entry cost and you are comfortable owning IP warm-up, authentication, and rotation yourself, and you do not need real inboxes for replies. Choose Google Workspace or Microsoft 365 if you want real, isolated mailboxes that handle replies and connect cleanly to common sequencers, and you accept per-mailbox pricing plus the operational work of provisioning and rotating domains. Between the two real-mailbox options, both are credible; the deciding factors are usually existing tooling, admin preferences, and how each provider’s policies treat your sending pattern.

Whichever you pick, the same physics apply: domains burn under sustained volume, rotation beats brute force, and infrastructure never substitutes for consent.

How Mailionaire approaches this

For transparency: Mailionaire provisions real, isolated Microsoft 365 tenants — one sending domain per tenant, up to 100 mailboxes — and is built and run in Switzerland. We set the DNS, monitor deliverability signals, and replace worn mailboxes and domains. Turn on the optional EU/Swiss residency add-on and your mailbox content sits at rest in the EU or Switzerland under the Microsoft EU Data Boundary, on EU and CH IPs. Pricing is flat: $50 per month per active domain, with no per-mailbox metering and no year-two price shock, plus a 14-day money-back guarantee. The same physics still apply, including consent law: infrastructure handles where your data lives and how mail is delivered, not whether you have permission to send. You can read more about the underlying setup on our European email infrastructure page.

FAQ

Is Microsoft 365 better than Google Workspace for cold email?

Neither is universally better. Both give you real, isolated mailboxes tied to your own domain. The right pick depends on your tooling, admin preferences, and how each provider's policies treat your sending pattern.

Can raw SMTP replace real mailboxes for cold outreach?

Rarely on its own. SMTP relays are good at pushing volume, but cold outreach usually needs real inboxes to receive and manage replies, which most pure SMTP services do not provide.

Why do sending domains burn out?

Sustained cold-email volume degrades a domain's reputation with mailbox providers until mail starts landing in spam. This often happens within a few months and faster at higher daily volume. The common fix is to rotate several domains at modest per-mailbox volume.

Does using Microsoft 365 or EU infrastructure make cold email legal?

No. Where mail is hosted and which IPs you send from affect deliverability and data location, not legal permission. Many EU markets, including Germany under UWG §7(2), require opt-in even for B2B, and the sender is always responsible for consent and local law.

---