Outlook attachment errors usually have two different causes: the complete email is too large, or Outlook blocks the file type for security. The applicable size limit depends on the account, and Microsoft’s published Outlook.com guidance conflicts: one page says 20 MB, while a separate page says 25 MB. Here’s how to identify the issue and choose a safe next step.
First, identify whether the problem is size or file type
- Size problem: Outlook reports that the message or attachment is too large, or it will not send after you attach a large file. The limit generally applies to the whole email, not just the file.
- File-type problem: Outlook warns that the attachment is blocked or will not let you attach or open it. Outlook blocks some extensions because they can carry viruses.
If the message does not clearly identify the cause, try a small, ordinary document as a check. If that works but the original file does not, the original may exceed the size limit or have a blocked extension.
What size limit applies to an Outlook email?
Microsoft’s general guidance says internet email accounts such as Outlook.com and Gmail have a 20 MB email limit, while Exchange business accounts have a 10 MB default. Those figures include the email and its attachments, rather than measuring only the attached file. Microsoft says the guidance applies to Outlook for Microsoft 365, Outlook 2024 and 2021, Outlook.com, New Outlook for Windows, and Outlook Web App. See Microsoft’s guidance on reducing attachment size.
There is a specific discrepancy for Outlook.com: a separate Microsoft known-issue page states a 25 MB email limit, including text and inserted or attached files. That page, last updated 6 November 2023, also says OneDrive links do not count toward the email limit and linked files may be up to 2 GB. The 20 MB figure on Microsoft’s general page and the 25 MB figure on the Outlook.com page are not consistent. Neither should be treated as a universal, guaranteed Outlook.com limit; check current guidance or the limit enforced by your account. The dated statement is on Microsoft’s Outlook.com known-issue page.
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| Account or guidance | Published size figure | What it covers |
|---|---|---|
| Internet email accounts, including Outlook.com and Gmail; Microsoft general guidance accessed 7 October 2026 | 20 MB | Whole email, including attachments |
| Exchange business accounts; Microsoft general guidance accessed 7 October 2026 | 10 MB default | Whole email, including attachments; an organization may set a different limit |
| Outlook.com; Microsoft known-issue page last updated 6 November 2023 | 25 MB | Whole email, including text and inserted or attached files; conflicts with the general guidance |
| OneDrive-linked file on the same Outlook.com known-issue page, last updated 6 November 2023 | Up to 2 GB | Linked file; Microsoft says it does not count toward that page’s Outlook.com email limit |
Exchange’s 10 MB figure is a default, not a promise about every work or school mailbox. Administrators can control some Exchange attachment-security settings, so managed accounts may have different size limits or policies. If you use an organization account, ask its email administrator what applies to your mailbox.
Which file types does Outlook block?
Outlook blocks some file types when sending or receiving because they can contain viruses. Examples of potentially unsafe extensions include .exe, .bat, .vbs, and .js. These are examples, not a complete list: Microsoft’s blocked-attachments page includes many extensions, such as .apk, .cmd, .com, .iso, .pst, .py, .reg, .scr, .vhd, .vhdx, .wsf, and .xll.
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A blocked attachment is a security signal, not just a file-transfer inconvenience. If you were not expecting the file, verify it with the purported sender using a trusted channel. Do not open attachments from senders you do not trust. Microsoft’s attachment-opening guidance gives examples of unsafe types and advises caution.
What to do when an attachment is too large
Share a cloud link
Microsoft recommends sharing a link instead of attaching the full file. It names OneDrive, Dropbox, iCloud, and Google Drive as examples. Outlook’s general guidance also describes saving a file to a SharePoint library in New Outlook. A link keeps the file out of the email payload, but the recipient still needs permission to access it. Share it with the intended recipients and check that they can open it. For Outlook.com, Microsoft’s dated known-issue page specifically says OneDrive links do not count toward its stated email limit.
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Compress the file
Compressing a file may reduce its size enough to send as an attachment. It only solves the problem if the resulting file brings the entire email below the limit that applies to your account. Compression can change how a file is packaged; it does not make an untrusted file safe.
What to do when Outlook blocks the file type
Share the file through an approved cloud service
Microsoft’s general blocked-attachment guidance recommends using OneDrive and sharing a link. This avoids attaching the blocked file directly, but it does not establish that every service, recipient, or organization will permit access. Follow your employer’s file-sharing and security rules when using a managed account.
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Check with your administrator before trying another route
Microsoft says an Exchange administrator may unblock certain file types. If this is a work or school account, ask the administrator whether the file is permitted and which approved delivery method to use. Do not treat old registry instructions for Outlook 2010, 2013, or 2016 as general instructions for current Outlook clients; Microsoft’s older guidance has that historical version scope.
Use a ZIP archive only where policy permits
Microsoft’s Windows attachment-troubleshooting page suggests placing an unsafe file in a ZIP archive before attaching it. This is not a guarantee that Outlook, an organization’s mail gateway, or the recipient’s system will accept it. Do not use ZIP to evade a security policy, and do not open an archived file simply because it was deliverable.
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Why changing the filename is not a safety fix
A different filename does not change what a file does or whether it is safe. Do not rename an executable or script to make it appear harmless. Use an approved sharing method or ask the administrator to confirm how the file should be handled.
Choose the workaround that matches the error
| Problem | Best next step | What it does not guarantee |
|---|---|---|
| Whole email exceeds the applicable size limit | Share a cloud link, or compress the file if that reduces the complete message enough | A compressed file may still exceed the limit; a link still needs suitable recipient permissions |
| Outlook blocks the file extension | Use an approved cloud link or ask the Exchange administrator about policy | A ZIP archive or link does not override organizational security rules or make the file trustworthy |
| Work or school account has an unclear limit or block | Ask the email administrator about the mailbox’s actual size and attachment policy | The published Exchange default may not match the organization’s settings |
Microsoft’s general recommendations for oversized files are in Reduce attachment size to send large files with Outlook. For Windows-specific attachment issues, see Troubleshoot attachment problems in Outlook for Windows.
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