Exchange Online is Microsoft's hosted email service, delivering email, calendar, contacts and tasks to computers, the web and phones. Most small businesses don't buy it on its own, because it's the mail engine already inside Microsoft 365 Business Basic, Business Standard and Business Premium.
Here's the catch. The limits that bite aren't storage, they're sending: 10,000 recipients a day and 30 messages a minute per user, as of October 2026. And Microsoft says plainly that legitimate bulk commercial email, such as newsletters, belongs with a specialist provider, not on Exchange Online.
Where Exchange Online comes from
The service covers the Business plans (Basic, Standard and Premium), the Enterprise plans (E1, E3, E5 and F3) and the standalone Exchange Online Plan 1, Plan 2 and Kiosk. Each person who uses it needs a subscription and gets their own mailbox. Conference rooms and shared mailboxes don't need a subscription. One exception to watch: Microsoft 365 F1 doesn't include the right to an Exchange mailbox.
It also connects to Microsoft Entra ID, which handles the sign-in side. If you're comparing it with the other big option for a small team, my take on Gmail vs Outlook covers the everyday differences.
Mailbox sizes by plan
| Mailbox | Size, as of October 2026 |
|---|---|
| User mailbox, Business Basic, Standard and Premium | 100 GB |
| User mailbox, Office 365 E1 | 50 GB |
| User mailbox, F3 | 2 GB |
| Archive, Business Basic and Standard | 50 GB |
| Archive, Business Premium | 1.5 TB, auto-expanding |
| Shared mailbox | 50 GB, or 50 or 100 GB where a licence applies |
| Resource mailbox | 50 GB |
| Group mailbox | 50 GB |
| Public folder mailbox | 100 GB |
The archive is where Business Premium pulls ahead. 50 GB of archive on Basic and Standard is plenty for most people, but anyone who never deletes anything will find 1.5 TB auto-expanding a lot more comfortable.
The sending limits that matter
| Limit | Per user, as of October 2026 |
|---|---|
| Recipients per 24 hours, rolling | 10,000 |
| Messages per minute | 30 |
| Recipients per message | Up to 1,000, and an admin can set anything from 1 to 1,000 |
| Recipients per encrypted message | About 200 |
| Receiving, per mailbox | 3,600 messages per hour |
Two counting rules catch people out. A distribution group from the address book counts as one recipient, but a personal distribution list counts every member. So a contact list of 300 people saved in your own Outlook uses 300 of your 10,000, while a company distribution group counts once.
On the receiving side, no single sender is allowed more than a third of that 3,600 messages per hour. That stops one noisy system from filling a mailbox on its own.
The onmicrosoft.com trap
Every new tenant starts on a default onmicrosoft.com domain. Mail sent from that default domain is throttled to 100 external recipients per organisation in a rolling 24 hours, and anything over gets bounced with the code 550 5.7.236. That's for the whole organisation, not per person, so add your own domain before anyone starts emailing customers.
There's a second ceiling too. The Tenant External Recipient Rate Limit caps external recipients per day according to how many licences you hold, and a trial tenant is capped at 5,000 a day.
Why newsletters don't belong on Exchange
Microsoft tells customers who need legitimate bulk commercial email, such as newsletters, to use a specialist third-party provider. For high volumes to external recipients it recommends Azure Communication Services email instead. The 30-a-minute and 10,000-a-day limits are built for people writing email, not for a list send.
Protection and recovery you already have
Built-in anti-spam and anti-malware protection, quarantine, mailbox replication, single item recovery and deleted mailbox recovery are available across plans. The one gap: single item recovery isn't available on F3 and Kiosk.
One quirk with inbox rules. A message is only redirected or forwarded once by inbox rules, and if a second rule tries to redirect it again, it's dropped without a non-delivery report. So chained forwarding rules can lose mail silently.
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Getting Exchange Online set up right
- Check which plan you hold. Business Basic, Standard and Premium already include Exchange Online.
- Add your custom domain so mail stops going out from onmicrosoft.com.
- Keep newsletters and bulk mail off Exchange and use a dedicated sending service.
- Avoid chains of forwarding rules, since a second redirect is dropped.
- Check the limits table before you move a busy shared inbox across.
My verdict on Exchange Online
For everyday business email it's hard to fault: 100 GB mailboxes on every Business plan, built-in spam filtering and recovery, and it's already in the plan you're paying for. I'd move to Business Premium if archive space is a real squeeze. I'd never use it for newsletters, because Microsoft says not to and the 30-a-minute limit isn't built for list sends. If you're picking a plan, here's a breakdown of Microsoft 365 plans.
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Frequently asked questions
Is Exchange Online included in Microsoft 365 Business plans?
Yes. Business Basic, Business Standard and Business Premium all include Exchange Online.
How big is an Exchange Online mailbox?
As of October 2026, 100 GB on Business Basic, Standard and Premium, 50 GB on Office 365 E1 and 2 GB on F3.
How many emails can I send per day with Exchange Online?
Up to 10,000 recipients per user in a rolling 24 hours, at no more than 30 messages a minute.
Can I send newsletters from Exchange Online?
Microsoft tells customers who need legitimate bulk commercial email, such as newsletters, to use a specialist third-party provider.
Why is my mail from onmicrosoft.com being blocked?
Mail from the default onmicrosoft.com domain is limited to 100 external recipients per organisation in a rolling 24 hours. Add your own domain to lift it.
Do shared mailboxes need a licence in Exchange Online?
No subscription is needed for shared mailboxes or conference rooms. A shared mailbox holds 50 GB, or 50 or 100 GB where a licence applies.
Why did my forwarded email disappear?
Inbox rules only redirect or forward a message once. A second redirect is dropped without a non-delivery report.
