If Microsoft 365 / Office 365 messages are failing due to size limits, you can raise send and receive limits in Exchange Online. This guide shows the current defaults, maximums, and the exact steps in EAC or PowerShell.
Quick Answer (2026) — the reality
You can configure Exchange Online up to 150 MB, but most users will not be able to email a 150 MB file end-to-end.
- Classic Outlook (Windows / Classic Mac): up to 150 MB when configured.
- OWA (web): practical max ~112 MB due to encoding/overhead; may enforce lower in some cases.
- Mobile + New Outlook for Mac: 33 MB cap (raising server limits won’t change that).
- Even if you can send it, many recipients have lower receive limits—links avoid this.
- Best practice: if it’s over ~30 MB, use a OneDrive/SharePoint link (works everywhere and avoids recipient limits).
Source:Microsoft Learn: Exchange Online limits
Use OneDrive links for large files
- Sharing link: OneDrive/SharePoint supports files up to 250 GB (send a link).
- Cloud attachment (“Attach from OneDrive”) avoids classic attachment limits; exact size can vary by client and policy, and if it fails, use a sharing link.
- Classic attachment: OWA practical cap is 112 MB.
Source:Microsoft: OneDrive/SharePoint file size limits
Key takeaways
- Message size includes attachments and encoding overhead.
- There is no separate attachment limit above the message size limit.
- Attachment count limit: 250.
Microsoft 365 message size limits (defaults vs max)
| Setting | Default (Exchange Online) | Maximum | Where to change |
|---|---|---|---|
| Organization send/receive limit | 35 MB send / 36 MB receive | 150 MB | Set-TransportConfig (PowerShell) |
| Per-mailbox send/receive limit | Inherits org default | 150 MB | EAC > Recipients > Mailboxes / Set-Mailbox |
Client caps (what users actually see)
| Client | Typical limit | Notes |
|---|---|---|
| Outlook desktop (Windows) / Classic Outlook for Mac | 150 MB | Matches max tenant setting. |
| Outlook on the web (OWA) | 112 MB | OWA enforces a lower cap even if tenant is higher. |
| Outlook iOS / Android | 33 MB | Mobile apps are more restrictive. |
| New Outlook for Mac | 33 MB | Matches mobile-style limits. |
Source:Microsoft Learn: Exchange Online limits
Limits stack (effective size)
Effective limit = the smallest of: tenant transport limit (Set-TransportConfig) → mailbox limit (Set-Mailbox) → client limit (OWA/mobile/etc.) → recipient/server limit.
Keep transport and mailbox limits aligned—the smallest limit wins.
If users still can’t attach large files, check what client they’re using (New Outlook/mobile)—the client cap is usually the blocker, not Exchange.
Why 150 MB still fails
- Attachments are encoded, so the total message size is larger than the raw file.
- OWA is capped at 112 MB (practical max) because external routing/encoding overhead reduces usable size.
- OWA can also enforce a limit about 25% lower than your configured max (example: 100 MB configured → 75 MB in OWA).
- Outlook mobile apps and New Outlook for Mac are limited to 33 MB.
- Large distribution groups (5,000-99,999 recipients) cap messages at 25 MB.
- Recipients or external servers may enforce lower limits.
- Encryption scenarios can have lower size limits in some tenants.
Method 1: Exchange admin center (EAC)
Use EAC for mailbox-specific or bulk updates. Organization-wide transport limits are set via PowerShell.
EAC method 1: Single mailbox (or selected mailboxes)
- Go to Recipients > Mailboxes.
- Select a mailbox and open Message size restrictions (View details).
- Set Max send/receive size.
- Save and wait for replication.
EAC method 2: Bulk edit for multiple mailboxes
- Select multiple mailboxes in Recipients > Mailboxes.
- Choose Bulk edit > Message size restrictions.
- Update the send/receive limits and save.
Changes can take a few minutes to apply across Microsoft 365.
Method 2: PowerShell (Exchange Online)
This method is best for bulk updates or automation.
Connect to Exchange Online
Install-Module ExchangeOnlineManagement -Scope CurrentUser Import-Module ExchangeOnlineManagement Connect-ExchangeOnline -UserPrincipalName admin@domain.comCheck current limits
Get-TransportConfig | Format-List MaxSendSize,MaxReceiveSize Get-Mailbox user@domain.com | Format-List MaxSendSize,MaxReceiveSizeSet organization-wide limits
Set-TransportConfig -MaxSendSize 150MB -MaxReceiveSize 150MBYou can set send and receive sizes independently (MaxSendSize / MaxReceiveSize).
Set a single mailbox
Set-Mailbox user@domain.com -MaxSendSize 150MB -MaxReceiveSize 150MBSet multiple mailboxes
@("user1@domain.com","user2@domain.com") | ForEach-Object { Set-Mailbox $_ -MaxSendSize 150MB -MaxReceiveSize 150MB }Set all mailboxes (bulk)
Warning: This updates every mailbox in the tenant. Test on 1-2 mailboxes first.
Get-Mailbox -ResultSize Unlimited -RecipientTypeDetails UserMailbox | Set-Mailbox -MaxSendSize 150MB -MaxReceiveSize 150MB -Confirm:$falseSet defaults for future mailboxes (Mailbox Plans)
Get-MailboxPlan | Set-MailboxPlan -MaxSendSize 150MB -MaxReceiveSize 150MBDo this in one go (org + plans + users, run in a fresh PowerShell session)
Set-TransportConfig -MaxSendSize 150MB -MaxReceiveSize 150MB Get-MailboxPlan | Set-MailboxPlan -MaxSendSize 150MB -MaxReceiveSize 150MB Get-Mailbox -ResultSize Unlimited -RecipientTypeDetails UserMailbox | Set-Mailbox -MaxSendSize 150MB -MaxReceiveSize 150MB -Confirm:$falseVerify after changes
Get-TransportConfig | Format-List MaxSendSize,MaxReceiveSize Get-Mailbox user@domain.com | Format-List MaxSendSize,MaxReceiveSizeDisconnect
Disconnect-ExchangeOnlineTroubleshooting common errors
- External recipient rejection (common): Even if your tenant and client allow large messages, the recipient’s mail system may have a much lower limit (often ~20-25 MB). You can’t change the recipient’s server limits—use a OneDrive/SharePoint link instead.
- 552 5.2.3 message size exceeds fixed maximum: Raise limits in EAC/PowerShell and retry.
- 5.3.4 Message size exceeds fixed maximum message size: Lower the attachment size or raise limits.
- 4.3.1 Insufficient system resources: Large attachments can trigger this; retry later or use OneDrive links.
- Outlook says the attachment is too large: The client may have a lower limit; use OneDrive links.
Frequently Asked Questions
What is the maximum email size in Microsoft 365 (Office 365)?
Exchange Online supports up to 150 MB, but default limits are 35 MB send / 36 MB receive and client or recipient limits can be lower.
What are the Outlook and OWA attachment limits?
Outlook desktop/Classic Mac supports 150 MB, OWA is 112 MB, and Outlook iOS/Android plus New Outlook for Mac are 33 MB.
How do I check the current message size limits?
Run Get-TransportConfig and Get-Mailbox in Exchange Online PowerShell to view MaxSendSize and MaxReceiveSize.
Can I increase the email size limit for one user?
Yes. Use EAC for a mailbox, or run Set-Mailbox with MaxSendSize and MaxReceiveSize.
Why does a 150 MB attachment still fail?
Attachments are encoded, so the total message size is larger than the raw file. OWA caps are lower and can be 25% below your configured limit, and recipient limits can also block delivery.
Does increasing the Office 365 limit guarantee delivery?
No. Recipients can reject messages if their own limits are smaller.
The Final Word
Microsoft 365 (Office 365) size limits can be raised up to 150 MB, but large attachments still face encoding overhead, client caps, and recipient limits. Use EAC for quick changes or PowerShell for bulk updates, and share large files with OneDrive or SharePoint when possible.
