How to Increase the Attachment Size Limit in Outlook and Exchange (2026 Guide)

Email was never really designed for large files, and Microsoft's default limits reflect that. If you or your users keep hitting a "file size exceeds the allowable limit" error, the fix depends entirely on which Microsoft email platform you're using — on-premises Exchange Server, Exchange Online (Microsoft 365), or a personal Outlook.com account.

This guide covers all three, plus the client-side caps that can quietly override your server settings even after you've raised them.

Message Size vs. Attachment Size: What's the Difference?

These two terms get used interchangeably, but they're not the same thing:

• Message size limit applies to the entire email — header, body, and all attachments combined.
• Attachment size limit applies to a single attached file.

When people say "increase the attachment size limit," they're almost always really adjusting the maximum message size, since that's the setting Exchange actually exposes. Keep in mind that attachments are Base64-encoded for transport, which inflates their size by roughly 33% — so a 25 MB file can grow to over 33 MB by the time it's actually transmitted. Any limit you set should account for that overhead.

Option 1: On-Premises Exchange Server (2016/2019)

The default maximum message size on an on-premises Exchange server is 10 MB. Most organizations raise this to 25 MB or higher; Exchange allows values up to a much larger ceiling, or unlimited, though an unrestricted limit can cause delivery problems with mail servers that can't handle very large messages.

Steps (Exchange Admin Center):

1. Open the Exchange Admin Center (ECP).
2. In the left menu, select Mail Flow.
3. Select Receive Connectors.
4. Click the three-dot menu (⋯) for more options.
5. Select Organization Transport Settings.
6. Under Limits, adjust Maximum receive message size (MB) to the value you need — 25 MB is a common baseline.
7. Adjust Maximum send message size (MB) as well, if needed.
8. Click Save.

You can also manage this through the Exchange Management Shell if you need to apply limits to specific connectors, servers, or individual mailboxes rather than the whole organization — useful if only a subset of users need higher limits.

Option 2: Exchange Online (Microsoft 365)

Exchange Online works differently from on-premises Exchange, and its defaults are lower than most people expect:

• Default send limit: 35 MB
• Default receive limit: 36 MB
• Maximum you can configure: 150 MB total message size (available on Business Premium, E3, E5, and similar plans)

Steps (Exchange Admin Center):

1. Go to the Microsoft 365 admin center, then open the Exchange Admin Center.
2. In the left pane, select Recipients, then Mailboxes.
3. Select the mailbox you want to adjust.
4. Open Mail flow settings, then Message size restrictions.
5. Set your desired Maximum send size and Maximum receive size (up to 150 MB).
6. Save your changes.

To apply the change organization-wide rather than per mailbox, use PowerShell:

Connect-ExchangeOnline
Set-TransportConfig -MaxSendSize 150MB -MaxReceiveSize 150MB

Changes can take some time to propagate across Microsoft's servers, so don't be surprised if it takes a little while before the new limit is fully in effect.

Option 3: Personal Outlook.com Accounts

If you're using a free, personal Outlook.com or Hotmail account rather than a business or school account, there's no admin panel to adjust — Microsoft sets a fixed limit of 25 MB per message (attachments and body combined), and it can't be raised. For anything larger, you'll need to share a file via OneDrive link instead of attaching it directly.

The Client-Side Trap: Why Raising the Server Limit Isn't Always Enough

This is the part that catches a lot of admins off guard: even after raising your Exchange or Microsoft 365 limits, users can still hit errors — because the client they're using enforces its own, separate cap:

In other words, raising your Exchange Online limit to 150 MB won't help a user attaching a 60 MB file from their phone — the mobile app caps out at 33 MB regardless of server settings.

Best Practice: Use Cloud Attachments for Large Files

Rather than chasing higher and higher size limits, it's usually more reliable to share large files as cloud attachments — a link to a file stored in OneDrive or SharePoint rather than the file itself. This approach:

• Avoids encoding overhead and message-size limits entirely
• Supports files up to 250 GB
• Works consistently across desktop, web, and mobile clients
• Lets recipients access the latest version of a file rather than an outdated copy

In Outlook, this is usually as simple as selecting Attach File > Browse Web Locations and choosing the file from OneDrive, rather than attaching it directly from your computer.

Verifying Your Current Limits

To check your current settings in Exchange Online via PowerShell:

Get-TransportConfig | Select MaxSendSize, MaxReceiveSize
Get-Mailbox -Identity user@domain.com | Select MaxSendSize, MaxReceiveSize

For on-premises Exchange, the equivalent commands are:

Get-ReceiveConnector | Format-Table Name, Max*Size
Get-TransportRule | Where {($_.MessageSizeOver -ne $null)} | Format-Table Name, MessageSizeOver

Common Pitfalls

• Only raising the server limit and not checking client caps — mobile and OWA users will still fail on large files even after a server-side increase.
• Setting an unlimited or extremely high limit on-premises — this can cause deliverability issues with external recipients whose mail servers reject oversized messages.
• Forgetting that recipient limits matter too — even if your outgoing limit is 150 MB, the message will bounce if the recipient's mail server enforces a lower incoming limit.
• Not accounting for Base64 encoding overhead — always set your limit meaningfully higher than the actual file size you want to support.

Final Thoughts

Whether you're managing on-premises Exchange, Exchange Online, or supporting users on personal Outlook.com accounts, the underlying goal is the same: match your limits to how people actually work, without creating delivery problems on the other end. For most organizations, a moderate server-side increase (25–50 MB) combined with encouraging OneDrive/SharePoint links for anything larger strikes the best balance between usability and reliability.