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Outlook attachment problems usually have two different causes: the message is too large, or Outlook blocks the file type for security. Size limits depend on the email service and account settings; Microsoft’s published Outlook.com figures even conflict. Check which issue you have before choosing a workaround.
Is Outlook rejecting the file because of its size or its type?
Look at the error message and the file involved. A size error means the email, including its attachments and message content, exceeds the limit for that account. A blocked-type warning means Outlook or an organization’s security policy does not allow that file extension. These are separate restrictions, so compressing a file will not necessarily resolve a blocked-type warning, and changing the file type will not solve an oversized message.
Microsoft explains that Outlook blocks some file types to help protect users and recipients from viruses. Executable and script extensions such as .exe, .bat, .vbs, and .js are examples of potentially unsafe types. The examples are not a complete list: see Microsoft’s blocked-attachments list for the extensions it identifies.
What is Outlook’s attachment size limit?
The limit applies to the whole email, not just the attachment. Microsoft’s general guidance gives a 20 MB limit for internet email accounts such as Outlook.com and Gmail, and a 10 MB default for Exchange business accounts. Microsoft says the figures include the email and attachments. The Exchange figure is a default, not a universal cap: an organization can set different limits.
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Outlook.com has a documentation discrepancy. Microsoft’s general large-file guidance lists 20 MB for internet email accounts including Outlook.com, while a separate Outlook.com known-issue page says 25 MB, including message text and inserted or attached files. That page was last updated on 6 November 2023. Neither figure should be treated as a guaranteed universal limit for every Outlook.com account. Check the current guidance and the limit enforced by your account.
| Account or guidance | Published limit | What the figure covers |
|---|---|---|
| Internet email accounts, including Outlook.com and Gmail (Microsoft’s general guidance) | 20 MB | The whole email, including attachments. Source: Microsoft Support. |
| Exchange business accounts (Microsoft’s general guidance) | 10 MB default | The whole email, including attachments; an administrator may configure a different limit. Source: Microsoft Support. |
| Outlook.com (separate known-issue page, last updated 6 November 2023) | 25 MB | Message text and inserted or attached files. Source: Microsoft Support. |
These are documented figures, not a promise that every message at that size will send: an account’s settings and the receiving mail service can affect what succeeds. Microsoft’s general guidance covers Outlook for Microsoft 365, Outlook 2024 and 2021, Outlook.com, New Outlook for Windows, and Outlook Web App. For a work or school account, ask your administrator which limits apply.
How to send a file that is too large
Microsoft recommends sharing a cloud-storage link or compressing the file. The right choice depends on whether you need to attach the file itself or share access to a stored copy.
Share a cloud link
Upload the file to a cloud service and share a link with the intended recipients. Microsoft names OneDrive, Dropbox, iCloud, and Google Drive as examples; New Outlook guidance also describes saving a file to a SharePoint library. The email then contains a link rather than the full file, so the file does not count as an attachment in the same way. Set access so the intended recipients can open it, and do not assume every recipient or organization permits every sharing service.
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For Outlook.com specifically, Microsoft’s known-issue page, last updated 6 November 2023, says a OneDrive link does not count toward the email size limit and that a linked file may be up to 2 GB. Treat those figures as guidance from that dated support page, not an unchanging guarantee for all accounts or sharing scenarios.
Compress the file
Compressing a file into an archive can reduce its size, but only helps if the resulting whole email fits the applicable limit. Some files, such as already-compressed media, may shrink little. A ZIP archive is also mentioned in Microsoft’s Windows troubleshooting guidance for certain blocked attachments, but an organization’s mail-security rules may still reject it.
Why does Outlook block certain file types?
Some extensions can run code or otherwise carry security risks. Microsoft says Outlook blocks sending and receiving certain types to help protect users and recipients; its list includes examples such as .exe, .bat, .cmd, .scr, .vbs, .py, and .reg, among many others. The applicable behavior may also depend on the Outlook version and an organization’s mail policy.
A block is a security signal, not just a nuisance. If you were not expecting the attachment, confirm it with the purported sender using a trusted channel before opening or sharing it. Microsoft’s guidance on opening, saving, and editing Outlook attachments cautions users about potentially unsafe files, particularly those from unknown or untrusted senders.
What to do when a file type is blocked
- Verify the file first. If the attachment is unexpected, contact the sender through a trusted channel. Do not open it merely because you found a way to deliver it.
- For a legitimate file, use a cloud link where appropriate. Microsoft recommends OneDrive for sharing a blocked attachment; use a service and access settings that comply with your organization’s policy.
- For a work or school account, ask the administrator. Microsoft notes that Exchange administrators may unblock certain file types. The administrator can tell you whether an approved method exists for your organization.
- Do not rely on renaming the file. Changing an extension does not make a file safe, and it may not satisfy the recipient’s security controls.
Microsoft’s older guidance about “Level 1” and “Level 2” attachment categories applies to Outlook 2010, 2013, and 2016. Its detailed registry procedure is version-specific and should not be treated as a general fix for current Outlook clients.
Quick troubleshooting checklist
- The error mentions size: identify whether the account is internet mail or Exchange, remember the limit covers the whole message, then share a cloud link or compress the file.
- The error names an unsafe or blocked extension: check Microsoft’s blocked-attachments list, verify the file’s legitimacy, and use an approved sharing method or ask the administrator.
- The attachment appears to send but the recipient cannot access it: check that the link’s sharing permissions include the intended recipient and that their organization allows the service.
- The same file works in one account but not another: compare the account type and, for managed accounts, ask about organization-specific limits and policies.
For additional Windows-specific checks, see Microsoft’s Outlook attachment troubleshooting guide. Its older version-specific blocked-attachment article is limited to Outlook 2010, 2013, and 2016.
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