What is the maximum attachment size in Outlook.com?

It is 25MB per email. For larger files, upload them to OneDrive and share a link.

Sometimes I cannot send even a 20MB attachment in the Outlook desktop app.

The app's default limit is 20MB, and attachments become about 33% larger through Base64 encoding during transmission. Reduce the file size further or switch to a link.

What is the maximum size allowed in Microsoft 365 (Exchange Online)?

Administrators can increase the policy to allow up to 150MB. Tenant, mailbox, and connector limits must be adjusted consistently.

The recipient's mail server rejects the message.

The receiving server may have a smaller message limit. Share large files through a OneDrive or SharePoint link.

How can an administrator change the message size limit?

Increase organization or mailbox limits in the Exchange Admin Center, or adjust them with PowerShell (Set-Mailbox and Set-TransportConfig).

CONTENTS

Outlook Attachment Size Limits Explained (Outlook.com · Microsoft 365 · Exchange Server)

Exceeding the attachment size limit is a common reason emails fail to send. This guide covers the current limits for Outlook.com, Outlook desktop, Microsoft 365 (Exchange Online), and on-premises Exchange, along with solutions for oversized attachments (cloud links, compression, and splitting) and administrator settings.

Quick Fix: Resolve It in 3 Minutes

Switch to a OneDrive link (share a link instead of attaching the file)
Compress or resize the file and send it again
Split the file or share it through Teams or SharePoint

Limits by Service

Service	Default Limit (Typical Value)	Can It Be Increased?	Notes
Outlook.com	25MB attachment per email	—	Use a OneDrive link for large files
Outlook desktop (Internet account)	20MB by default	Can be changed through the registry	The app’s default limit
Exchange Online	Depends on organization/mailbox policy	Up to 150MB	Administrator configuration required
Exchange Server (On-premises)	10MB organization default	Can be increased by policy	TransportConfig/connectors/mailboxes

Why Does Sending Fail? The 33% Base64 Overhead

Attachments are Base64-encoded during transmission, making them about 33% larger than the actual file. For example, an 18MB file can become an email of about 24MB and exceed a 20–25MB limit. To be safe, aim for 70% or less of the limit.

Solutions for Users

Switch to OneDrive link sharing
Compress or resize images and videos, and optimize PDFs
Create a split archive and send it in multiple emails

Solutions for Administrators

Exchange Admin Center (EAC)

Increase the organization message size limit
Adjust MaxSendSize/MaxReceiveSize for individual mailboxes

PowerShell Example

Set-Mailbox -Identity user@contoso.com -MaxSendSize 50MB -MaxReceiveSize 50MB
Set-TransportConfig -MaxSendSize 50MB -MaxReceiveSize 50MB

Frequently Asked Questions

Over the 25MB Outlook.com limit? Use a OneDrive link.
Why does Outlook desktop fail with a 19MB file? The Base64 overhead can push it over the limit.
Microsoft 365 150MB? An administrator must increase the setting.
Rejected by the recipient’s server? The recipient-side limit or equipment may be involved, so sharing a link is safer.

Sources: Microsoft Support/Docs (Outlook.com, Outlook app defaults, Exchange Online/Server message size and attachment limits, and the 150MB announcement)

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Outlook Attachment Size Limits: 25MB, 20MB, 150MB, and Solutions