Outlook does not have one universal attachment limit. A 20 MB client setting applies to classic Outlook accounts using POP3, IMAP or HTTP; Exchange Server, Microsoft 365, Outlook on the web and mobile apps enforce different mailbox, organization, client and routing limits. You can change the classic Internet-account setting yourself, but an Exchange or Microsoft 365 policy must be changed by an administrator.
Use the account-specific instructions below. If the recipient’s system or an Exchange transport rule still imposes a lower ceiling, the reliable solution is to send a cloud-sharing link rather than forcing a larger attachment.
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First identify which Outlook limit you are hitting
Attachment size is governed by the account and the path the message takes, not only by the Outlook window where you add a file. Before changing anything, determine whether the account is an Internet account or an Exchange-based account.
Classic Outlook with POP3, IMAP or HTTP
For these Internet email accounts, classic Outlook has a documented default maximum of 20 MB for the total attachment payload. The limit applies to the sum of all attachments, not just one file. Outlook displays an error when the combined files exceed that client setting.
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Exchange Server
Traditional on-premises Exchange mailboxes default to a 10 MB maximum-send setting under the administrator’s configuration. A user cannot raise that server limit by editing Outlook on one computer.
Microsoft 365 and Exchange Online
Exchange Online combines mailbox-plan defaults, administrator settings, client ceilings and transport rules. The effective limit can therefore be lower than the largest value advertised for a plan. Outlook desktop, Outlook on the web and Outlook mobile do not necessarily accept the same message size.
How to confirm the account type
In classic Outlook, open File, choose Account Settings, then choose Account Settings again. The account list identifies whether the profile uses POP3, IMAP, HTTP, Exchange or Microsoft 365. This check matters because the registry procedure only changes the local check used by Internet accounts. It cannot turn an Exchange mailbox into an unrestricted Internet account.
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| Approach | Account or scope | Who can change it | What the recipient gets | Important constraint |
|---|---|---|---|---|
Registry value MaximumAttachmentSize |
Classic Outlook Internet account on one Windows profile | The Windows user, if permitted to edit the registry | Normal file attachment | Does not override an ISP, recipient, Exchange or organization limit |
Set-Mailbox message limits |
One Exchange Server or Exchange Online mailbox | Exchange administrator | Normal file attachment | Organization, connector, transport and client limits can remain lower |
| Mailbox-plan or organization configuration | Multiple Microsoft 365 mailboxes or an entire tenant | Microsoft 365 or Exchange administrator | Normal file attachment | Plan maximums and service tables still apply |
| Cloud-sharing link | Any account when attachment policy cannot be raised | The sender controls file sharing permissions | A link to the file | Recipient needs access to the shared location; it is not an email attachment |
Think of these as separate control points. The registry value affects one Windows profile. A mailbox setting affects one mailbox. Organization, connector and transport settings can affect many mailboxes. A sharing link avoids carrying the file through the email transport path at all. The method with the broadest scope is not automatically the best choice: raising a tenant-wide limit can create a policy and storage concern, while a link may be simpler for one occasional large file.
Increase the limit in classic Outlook for an Internet account
This procedure changes Outlook’s local client allowance. It is appropriate for POP3, IMAP or HTTP accounts in classic Outlook on Windows. It does not change a server’s maximum message size.
Before you edit the registry
- Close Outlook completely, including any Outlook process still running in Task Manager.
- Confirm that the account is an Internet account rather than Exchange or Microsoft 365. In Outlook, open File → Account Settings → Account Settings and check the account’s type.
- Create a registry backup. In Registry Editor, select the key you will edit, choose File → Export, and save the
.regfile somewhere you can retrieve it. - Use an amount that your mail provider and recipients can actually accept. Raising the local number cannot bypass a remote policy.
Set MaximumAttachmentSize
- Press Windows key + R, type
regedit.exe, and press Enter. Approve the User Account Control prompt if Windows asks. - Browse to
HKEY_CURRENT_USERSoftwareMicrosoftOffice<x.0>OutlookPreferences. - If your organization manages Office through policy, the corresponding policy location is
HKEY_CURRENT_USERSoftwarePoliciesMicrosoftOffice<x.0>OutlookPreferences. A policy value can take precedence over the normal user preference. - Right-click an empty area in the right pane, choose New → DWORD (32-bit) Value, and name it
MaximumAttachmentSize. If it already exists, double-click it instead. - Choose Decimal and enter the total attachment limit in kilobytes. For example, 20 MB is 20,480 KB and 50 MB is 51,200 KB when using 1 MB = 1,024 KB for the registry calculation.
- Click OK, close Registry Editor, and restart Outlook.
Use the correct Office version key
Microsoft identifies 16.0 as the Office version key for Microsoft 365, Office 2016, Office 2019 and Office LTSC 2021. Office 2013 uses 15.0. Replace <x.0> in the path with the key that matches the installed Office generation.
Worked registry examples
If you want a 50 MB local allowance, enter 51200 as a decimal DWORD value. That value represents the combined size of all attachments, not 50 MB per file. If you attach three files of 18 MB each, their 54 MB total exceeds that setting even though no individual file is larger than 50 MB. Conversely, a single 45 MB file and a 3 MB file total 48 MB and remain below the 50 MB client value, although encoding and server limits may still cause rejection.
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Changing the value does not make Outlook reconfigure your mail provider. It simply permits the client to attempt the send. If a message is rejected only after you click Send, record the exact error and ask the provider about its maximum message size rather than repeatedly increasing the DWORD.
What this change can and cannot do
The value controls Outlook’s local Internet-account attachment check. It does not increase the maximum message size accepted by your email host, your Internet service provider, the recipient’s system or an Exchange organization. If Outlook accepts the file but the message is rejected later, the server-side limit—not the registry value—is the cause.
Have an Exchange administrator raise the mailbox limit
For Exchange Server or Exchange Online, the administrator can set maximum send and receive sizes for an existing mailbox. The following Exchange PowerShell command sets both values to 50 MB for one mailbox:
Set-Mailbox -Identity "[email protected]" -MaxSendSize 50 MB -MaxReceiveSize 50 MBReplace the identity with the real mailbox address or alias. The administrator must connect to the appropriate Exchange environment and have permission to run Set-Mailbox.
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After changing the mailbox, verify what Exchange reports:
Get-Mailbox -Identity "[email protected]" | Format-List MaxSendSize,MaxReceiveSizeMaxSendSize controls messages the mailbox can send; MaxReceiveSize controls messages it can receive. These values are only one layer of enforcement. An organization-wide transport setting, connector, recipient restriction or client limit can still reject a message that is below the mailbox value.
What the administrator must check
Ask for the effective value, not merely the value someone intended to configure. The mailbox may report 50 MB while an organization, connector or transport rule allows less. The recipient’s mailbox and gateway can also enforce a smaller maximum. Administrators should therefore test both directions: send a message near the intended size and receive a similarly sized message from an external sender. A successful internal test does not prove that external routing will accept the same payload.
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Understand Microsoft 365 and Exchange Online ceilings
Exchange Online mailbox-plan documentation lists a default MaxSendSize of 35 MB and a default MaxReceiveSize of 36 MB. Where the subscription permits it, the maximum configured value is 150 MB. These are message-size settings, not a promise that every Outlook client can send a 150 MB file.
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Email attachments are encoded for transport, increasing the message by approximately 33 percent. A file that appears to be 30 MB can therefore produce a message of roughly 40 MB before other headers and content are counted. Leave headroom between the file size and the configured message limit; do not set a limit equal to the exact size of the file you want to send.
For planning, multiply the combined attachment size by about 1.33. A 25 MB group of files may produce a message around 33.25 MB, while a 50 MB group may produce one around 66.5 MB. These are estimates, not guarantees, because message text, headers, routing and translation can add more. If the message leaves Microsoft datacenters, translation encoding can create a further increase.
Client-specific Microsoft 365 limits
| Client or service path | Documented message and single-attachment ceiling | Qualification |
|---|---|---|
| Outlook desktop on supported Microsoft 365 plans | 150 MB | Service-table maximum; the mailbox, organization, recipient and routing path can impose lower values. |
| Outlook on the web | 112 MB | Lower than the service maximum because the web client and encoding limits reduce the sendable size. |
| Outlook mobile | 33 MB | Mobile message-size ceiling; it can be lower than the desktop allowance for the same mailbox. |
New Outlook for Mac and Outlook mobile honor the lowest applicable values among MaxAttachmentSize, MaxSendSize and MaxRequestSize. Microsoft also notes that clients may impose lower limits than the service maximum. Messages routed outside Microsoft datacenters can incur the same approximately 33 percent translation-encoding increase, further reducing the payload that can be sent successfully.
Why the same mailbox behaves differently on different devices
A desktop client may accept a message that Outlook on the web or a phone refuses because each client applies its own request or attachment ceiling. Switching clients is useful as a diagnostic, but it does not raise the mailbox or organization limit. If desktop Outlook succeeds and mobile fails, the result points to a client-specific ceiling. If every client fails at a similar size, investigate the mailbox, organization, recipient or routing policy.
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Why a larger setting can still produce “attachment too large”
When a message fails after you raise a value, check every limit in the route rather than changing the registry repeatedly.
- The attachments are being totaled. Several files whose combined size exceeds the limit fail just like one large file.
- Base64 encoding pushes the message over the ceiling. Compare the encoded message size, not only the file sizes shown in File Explorer.
- The client has a lower cap. Outlook on the web and Outlook mobile have lower documented ceilings than supported desktop Outlook plans.
- The mailbox value is higher than the organization value. Transport rules, connectors and organization-level settings can impose a stricter maximum.
- The recipient’s system rejects the message. Your account may accept the message while the recipient’s mail host, gateway or mailbox refuses it.
- An external route adds translation overhead. Messages leaving Microsoft datacenters can grow during encoding.
- A policy key overrides your preference. If the policy registry path contains
MaximumAttachmentSize, changing the normal Outlook preferences key may have no effect. - The account is not an Internet account. The registry method does not raise Exchange or Microsoft 365 limits.
Use compression carefully
Compressing files into a ZIP archive can reduce the payload when the contents are text, spreadsheets, uncompressed images or other data with redundancy. Videos, JPEG images, many PDFs and files that are already compressed often shrink little. Compression cannot bypass a hard message-size policy: the encoded ZIP still has to fit every applicable limit.
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Check the archive after creating it, and make sure the recipient can open the format. For confidential material, protect the archive with a separately communicated password and follow your organization’s security policy. Compression also changes the recipient’s experience: they receive one archive and must extract it, so confirm that this is acceptable before sending.
Send a cloud-sharing link when the limit cannot be raised
Microsoft Support identifies a file-sharing service as a workaround when an attachment exceeds the permitted size. Upload the file to an approved cloud location, create a sharing link, set the least-permissive access that still works for the recipient, and place the link in the email. This avoids the email message-size ceiling because the file is downloaded from the sharing service rather than carried inside the message.
A link is also appropriate when you need to distribute a file to several people, update the file after sending, or avoid creating multiple encoded copies in mailboxes. It is not equivalent to an attachment: the recipient needs network access and permission to the shared location, and the link can stop working if the file is moved, deleted or access is revoked.
Check access before sending
- Confirm that the recipient is allowed to open the file and that the link is not restricted to your organization unless that is intentional.
- Prefer view-only access when editing is unnecessary.
- Set an expiration date or remove access after delivery when the service supports it.
- Send a short test link to an alternate account if the file is business-critical.
- Do not put passwords or sensitive access codes in the same email as the link.
A practical troubleshooting sequence
- Note the exact error and whether it appears while attaching, while sending, or after delivery is attempted.
- Check the account type in Outlook’s Account Settings.
- Add the attachment sizes together and leave at least one-third headroom for encoding when estimating a message limit.
- If it is an Internet account, confirm the
MaximumAttachmentSizepath, Office version key, decimal value and Outlook restart. - If it is Exchange or Microsoft 365, ask an administrator to verify
MaxSendSizeand the organization, connector and transport settings. - Try the same mailbox in another supported client only as a diagnostic; a successful test there indicates a client-specific ceiling, not that the organization limit has changed.
- If the recipient still rejects the message, use an approved sharing link or ask the recipient for the maximum accepted size.
What to ask your administrator
- What are the effective
MaxSendSizeandMaxReceiveSizevalues for my mailbox? - Is there an organization, connector or transport-rule limit below the mailbox value?
- Which Outlook clients and mobile apps are approved, and what client-specific caps apply?
- Does external routing or a recipient gateway reduce the usable payload?
- Is cloud sharing approved for this file, and what access or retention controls are required?
Frequently asked questions
How do I increase Outlook’s 20 MB attachment limit?
If the account is POP3, IMAP or HTTP in classic Outlook for Windows, edit the MaximumAttachmentSize DWORD under the appropriate Office version key and enter the total size in kilobytes. Restart Outlook afterward. This does not raise a mail server or recipient limit.
Can I send more than 20 MB from Microsoft 365?
Possibly. Exchange Online lists a 35 MB default maximum send size and supports up to 150 MB where the subscription permits it, but the mailbox, organization, client, encoding and recipient route determine the usable payload. Outlook on the web lists 112 MB and Outlook mobile 33 MB.
Can a Microsoft 365 administrator increase the limit?
An administrator can adjust mailbox settings with Set-Mailbox and can manage organization-level configuration, subject to the subscription maximum. The administrator cannot guarantee delivery above a recipient gateway, connector, transport or client limit.
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The 20 MB value applies to the combined attachments in classic Internet accounts. Encoding also adds approximately 33 percent to the message, and a policy or client may impose a lower effective value.
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Does changing the registry work for Exchange or Outlook.com?
No. The registry value addresses classic Outlook’s local Internet-account check. Exchange, Microsoft 365 and other hosted services enforce server-side settings that require administrator or provider action.
Is a ZIP file guaranteed to solve the problem?
No. Compression helps only when the contents compress effectively, and the encoded archive must still fit every applicable limit. Videos, JPEG images and already-compressed files may shrink very little.
What is the safest alternative to a rejected attachment?
Use an organization-approved cloud-sharing service, set the narrowest access that works, test the link, and remove or expire access when appropriate. The recipient receives a link rather than an email attachment.
Why can desktop Outlook send a file that Outlook mobile cannot?
Clients have different ceilings. Microsoft lists 150 MB for Outlook desktop on supported plans, 112 MB for Outlook on the web and 33 MB for Outlook mobile. The mailbox and routing limits still apply to all of them.
Frequently Asked Questions
What is Outlook’s default attachment limit?
Classic Outlook uses a 20 MB total attachment default for POP3, IMAP and HTTP accounts. Exchange Server commonly defaults to 10 MB, while Microsoft 365 limits depend on the plan, mailbox, client and route.
Can I raise the limit without administrator access?
Only the classic Internet-account client setting can be changed locally, using the MaximumAttachmentSize registry value. Exchange and Microsoft 365 mailbox or organization limits require an administrator.
How much headroom should I leave for encoding?
Plan for approximately 33 percent overhead. A 30 MB group of files can create a message around 40 MB before other content and routing overhead are counted.
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What should I do if the recipient still rejects a larger message?
Ask about the recipient’s maximum size or send the file through an approved cloud-sharing service. Your own mailbox setting cannot override the recipient’s gateway or mailbox policy.
The Bottom Line
Use the registry only for classic Outlook Internet accounts. For Exchange or Microsoft 365, an administrator must change the mailbox or organization policy, and the usable size is reduced by encoding, client and routing limits. When those limits cannot be aligned, share the file by an access-controlled cloud link.
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