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What mailbox providers actually require

Google and Yahoo ask for proof of identity, an easy way to stop, and almost no complaints. Microsoft requires the first. None of them says that sending slowly helps.

Guide4 min readiSyncSO · Team

The short version.

Google, Yahoo and Microsoft publish what they require from senders. Google and Yahoo ask for three things: prove who you are, make it easy to stop, and keep complaints close to zero. Microsoft requires the first and recommends the second.

None of them says that sending slowly gets a message delivered. We built SYNC to send slowly anyway, and this piece separates what the providers require from what we chose ourselves.

What the three providers ask.

Google
Who it applies toEveryone who sends to personal Gmail accounts. Extra rules from 5,000 messages a day.[1]
What it requiresSPF or DKIM for all senders. SPF, DKIM and DMARC for bulk senders, plus one-click unsubscribe on marketing mail.
Yahoo
Who it applies toAll senders, with extra rules for bulk senders.[2]
What it requiresSPF or DKIM for all. For bulk: both, DMARC, a working one-click unsubscribe, and unsubscribes honoured within two days.
Microsoft
Who it applies toDomains that send more than 5,000 messages a day to Outlook.com, Hotmail.com and Live.com.[3]
What it requiresSPF and DKIM that pass, and DMARC that aligns with one of them. Microsoft says it will reject mail that fails.

The rule about complaints comes with a number. Google states it like this:

“Keep spam rates reported in Postmaster Tools below 0.10% and avoid ever reaching a spam rate of 0.30% or higher.”
Google, Email sender guidelines[1]

Yahoo asks senders to stay below 0.3%.[2] At one complaint in a thousand messages, a sender who writes to people who never asked to hear from them has very little room.

What a mailbox is allowed to send.

SYNC sends from the user's own mailbox, so the mailbox's own limits apply as well.

Google Workspace
Limit2,000 messages a day per user, and 3,000 unique recipients a day, of which 2,000 outside the organisation.
Source[4]
Microsoft 365
Limit10,000 recipients a day per mailbox, and 30 messages a minute.
Source[5]

SYNC sends at most three messages to one account in any seven days, and never writes to the same person twice within 48 hours. A seller who works fifty accounts stays far below both limits. Microsoft also says Exchange Online is not meant for bulk mail and points bulk senders to other services.[5]

What we chose, and what we cannot claim.

  • 01The mail leaves from your own mailbox, under your own name. The providers' identity checks then apply to your domain, so your domain should have SPF, DKIM and DMARC set up.
  • 02A person approves before anything is sent. That is our rule. No provider requires it.
  • 03At most three messages a week to one account. That is also our rule. It keeps volume low, and low volume leaves fewer people who can complain.
  • 04Every message should offer a plain way to say stop, and a stop should hold across every sender who uses SYNC.

We cannot claim that the pace itself improves delivery. We found no provider document that says so. What the providers measure is authentication and complaints.

the other side

What speaks against this.

The published thresholds protect consumer inboxes. Google's rules cover personal Gmail accounts, and Microsoft's cover Outlook.com, Hotmail.com and Live.com.[1][3] Many business recipients use Google Workspace or Microsoft 365. There the company's own filters decide, and we found no published threshold.

So a sender can follow every rule here and still land in a company's spam folder. The rules are a floor.

Delivery is also a different question from permission. Whether you may write to someone at all is a matter of law, and nothing in this piece answers it. A lawyer is reviewing that question for us.

method

We read each provider's own page on 29 September 2026 and quote or paraphrase it. Limits can change without notice; Google says so on its limits page. This piece contains no measurements of ours.

How we measure →
sources
  1. [1]Google. Email sender guidelines. Gmail Help.read in the originalapplies to mail sent to personal Gmail accounts
  2. [2]Yahoo. Sender best practices. Yahoo Sender Hub.read in the original
  3. [3]Microsoft. Strengthening email ecosystem: Outlook's new requirements for high-volume senders. Microsoft Tech Community, 2 April 2025.read in the originalapplies to Outlook.com, Hotmail.com and Live.com
  4. [4]Google. Gmail sending limits in Google Workspace. Google Workspace Admin Help.read in the original
  5. [5]Microsoft. Exchange Online limits. Microsoft Learn.read in the original

Where this lives in the product

product→Our commitmentsThe pace SYNC keeps: at most three messages a week to one account.

Keep reading

case study→We asked our own product who should buy itWe ran SYNC's research with ourselves as the seller: 594 companies, 35 flagged as wrong matches, 244 dossiers of which most say wait. What it got wrong, and why this is weak evidence.guide→Mapping the buying group: who really decides?Published group sizes run from three or four people to thirteen or more, because they count different things. A way to map the group in seven roles, and what our own maps can and cannot show.perspective→What a human approval is worthResearch on automation says an approval step is weaker than it sounds. Why we keep ours, what our numbers show so far, and why we expect it to wear down.

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