Attachment upload size limitation (External only)

Question

We are experiencing an issue with Outlook Web App (OWA) on Microsoft Exchange Server 2019 related to attachment upload size when accessing OWA externally from the internet.

When users access OWA internally (from the office network / internal URL), they can successfully attach and send files larger than 1 MB without any issues.

However, when users access OWA externally (via the internet / external URL), they are unable to attach files larger than 1 MB. Attempting to attach files larger than 1 MB results in the following error message:

"The following file couldn't be attached"

Answer 1

Hi @Sardor Suvonov 

Thank you for reaching out to Microsoft Learn Q&A!  

I truly appreciate the details you’ve shared, as they help narrow down the research.   Based on your description, it appears that the issue of being unable to add attachments larger than 1MB when using an external URL is more likely related to external settings rather than the Exchange Server configuration itself. 

To better understand your situation, may I clarify the following: 

Do you have any external components such as a reverse proxy, Web Application Firewall (WAF), or load balancer in place? 
Please double-check these components, as external traffic usually passes through them, and they often have a default upload size limit of 1MB. 

If you don’t have these components, please review the web.config file. If needed, you can increase the relevant settings as shown in the image below. 

Reference: Can't add an attachment in Outlook on the web - Exchange | Microsoft Learn 

For additional insight and troubleshooting, I recommend capturing data through IIS logs, Event Viewer, and Browser Developer Tools, ... 

I hope this helps. If you have further questions or updates, please don’t hesitate to reply.

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Answer 2

The issue you are experiencing with Outlook Web App (OWA) on Microsoft Exchange Server 2019, where users can attach files larger than 1 MB internally but not externally, could be related to the configuration of the message size limits in your Exchange environment.

By default, the maximum message size for attachments in Exchange is often set to 10 MB, but there may be specific configurations or restrictions applied to external access that could be limiting the attachment size to 1 MB.

To troubleshoot this issue:

1. Check the Exchange settings: Ensure that the message size limit for external users is set correctly. You may need to adjust the settings in the Exchange Admin Center or through PowerShell to allow larger attachments for external access.
2. Network Configuration: Verify if there are any network devices (like firewalls or proxies) that might be imposing size limits on attachments when accessed externally.
3. Run a network trace: This can help identify if there are any errors during the attachment process when users are accessing OWA externally. Look for specific error messages that might indicate the cause of the issue.

If the problem persists, consider consulting with your Exchange administrator to ensure that all settings are appropriately configured for external access.