Ever hit send on an email, only to get bounced back with an error about the file being too big? That’s the Office 365 message size limit doing its job — even when it feels like the wrong moment.
By default, Microsoft caps most mailboxes well below what people assume. The good news is that admins can raise this ceiling, and in this guide, you’ll learn exactly how, along with the traps that catch even experienced IT teams.
Table of Contents
What Is the Office 365 Message Size Limit?
The Office 365 message size limit setting controls the maximum size — including attachments and formatting overhead — that an email can reach before Exchange Online rejects it.
This isn’t one fixed number. It depends on your tenant settings, the specific mailbox, and even which Outlook app someone is using. Exchange Online technically supports messages up to 150 MB, though the default limits are closer to 35 MB for sending and 36 MB for receiving.
That gap between “supported” and “default” is exactly why so many admins search for office 365 message size limits in the first place. The platform can handle more — you just have to turn the dial yourself.
Default vs. Maximum Message Size
Here’s a quick reference for how the numbers stack up.
| Setting | Value | Where It’s Configured |
| Default send limit | 35 MB | Exchange Admin Center |
| Default receive limit | 36 MB | Exchange Admin Center |
| Maximum possible limit | 150 MB | EAC or PowerShell |
| Historical original default | 25 MB | Legacy tenants |
Note that Microsoft lets administrators set the maximum message size anywhere from 1 MB up to 150 MB, but the default for existing mailboxes stays untouched unless someone changes it. In other words, upgrading Office 365 doesn’t automatically raise your ceiling — you have to do it manually.
How to Change the Office 365 Max Message Size (EAC)
Changing the office 365 max message size for your whole organization takes just a few clicks if you have Global Admin access.
- Sign in to the Exchange Admin Center.
- Go to Recipients > Mailboxes.
- Click Mailflow settings, then Message size restriction.
- Under Default message size restriction, update the send and receive values (in KB, up to 153600 KB for 150 MB).
- Save your changes.
This changes the default for new mailboxes. For mailboxes that already exist, you’ll need a separate bulk edit step, which we cover next.
Updating Existing Mailboxes in Bulk
- In EAC, select Recipients > Mailboxes.
- Highlight the mailboxes you want to update.
- In the Bulk Edit panel, open Message Size Restrictions.
- Enter your new send and receive limits, then apply.
Give the change time to propagate. Larger tenants may need to wait a while before every mailbox reflects the new office 365 message size limit setting.
How to Set Message Size Limits with PowerShell
If you manage a large tenant, PowerShell is faster than clicking through the console one mailbox at a time.
Connect to Exchange Online PowerShell, then run:
Set-Mailbox -Identity “user@yourdomain.com” -MaxSendSize 150MB -MaxReceiveSize 150MB
To apply the change to every mailbox at once:
Get-Mailbox -ResultSize Unlimited | Set-Mailbox -MaxSendSize 150MB -MaxReceiveSize 150MB
And to make sure new mailboxes inherit the same setting going forward:
Get-MailboxPlan | Set-MailboxPlan -MaxSendSize 150MB -MaxReceiveSize 150MB
Always test on one mailbox first. Running bulk PowerShell commands without a test pass is one of the fastest ways to create a support ticket avalanche.
Office 365 Attachment Size Limit Setting for Specific Users
Not every user needs the max. A finance team might need bigger allowances for spreadsheets, while general staff are fine at the default.
To set a custom office 365 attachment size limit setting for one person:
- Open EAC > Recipients > Mailboxes.
- Select the user, then open Mailflow settings.
- Adjust Message Size Restrictions for that mailbox only.
This targeted approach keeps your organization’s average message size lower, which helps with storage planning and transport performance.
Message Size Limits by Outlook Client
Raising the server-side office 365 message size limits doesn’t guarantee every app will cooperate. Each Outlook client enforces its own ceiling:
- Outlook desktop (Windows/Mac classic): up to 150 MB
- Outlook on the Web (OWA): roughly 112 MB
- New Outlook, Outlook for iOS and Android: around 33 MB
Pros of raising limits:
- Fewer bounced emails during migrations
- Better support for design, engineering, and legal teams
- Less reliance on third-party file-sharing tools
Cons of raising limits:
- Larger mailbox storage consumption
- Slower sync speeds on mobile devices
- Recipients may still reject the message anyway
Common Mistakes to Avoid
Even seasoned admins trip over the office 365 message size limit change process. Watch for these:
- Forgetting the recipient’s limit. You can raise your outbound cap, but if their inbox caps incoming mail lower, delivery still fails.
- Assuming one setting covers everyone. Tenant defaults, mailbox plans, and individual mailboxes can each have different values.
- Ignoring encoding overhead. Attachments grow roughly 33% larger once encoded for email transport, so a 100 MB file can exceed a 130 MB limit.
- Skipping propagation time. Changes don’t apply instantly across a large tenant.
When Email Isn’t the Right Tool for Big Files
Even at 150 MB, email was never built to be a file-transfer system. For genuinely large attachments — or for moving years of archived mail between platforms — a dedicated migration approach is safer and faster than stretching message size limits to their breaking point.
This is where a lot of IT teams eventually land: instead of fighting size restrictions email by email, they move bulk data directly between systems. SkyMigrate handles exactly this kind of transfer, whether you’re consolidating mailboxes, moving to Microsoft 365, or backing up years of correspondence without worrying about a single message ever hitting a size cap.
FAQs
What is the maximum message size in 365? The technical ceiling is 150 MB, but most tenants default to 35 MB for sending and 36 MB for receiving. Admins can raise this through the Exchange Admin Center or PowerShell, though client apps and recipient servers may still enforce lower limits.
How do I send a 200 MB file in Outlook? You can’t send a 200 MB file as a direct attachment since it exceeds Exchange Online’s 150 MB cap. Instead, upload the file to OneDrive or SharePoint and share a link in your email, which avoids size restrictions entirely.
Does Office 365 support larger email messages up to 150 MB? Yes. Microsoft increased the maximum supported message size to 150 MB, giving admins flexibility to set limits anywhere between 1 MB and 150 MB per mailbox, though the default remains lower unless changed manually.
How do I send a 200 MB file on email? Traditional email attachments can’t handle 200 MB reliably across providers. Use a cloud storage link (OneDrive, SharePoint, or Google Drive) instead, which lets recipients download the file without triggering any message size limit.
What’s the Exchange Online default message size limit? Exchange Online defaults to a 35 MB send limit and 36 MB receive limit for most tenants. These can be changed by a Global Administrator through the Exchange Admin Center’s message size restriction settings.
Conclusion
The Office 365 message size limit isn’t as fixed as it looks. With the right admin access, you can raise it from the modest 35/36 MB default up to Microsoft’s 150 MB ceiling — but only if you account for client limits, recipient servers, and encoding overhead along the way.
For everyday large attachments, adjusting the office 365 message size limit setting through EAC or PowerShell will solve most problems. For genuinely large-scale data movement — mailbox consolidations, platform switches, or full email archives — that’s a job better suited to a proper migration tool than to email itself. If that’s where you’re headed, SkyMigrate’s team can help you move everything over cleanly, without a single size error along the way.
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