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Why Large Exchange Online Emails Are Delivered Late & How to Fix It
Question details
Users experience delivery delays when sending large emails to Exchange Online recipients due to network transport, security processing, or throttling.
- Product
- Exchange Online / Microsoft 365
- Device & OS
- not provided
- Scenario
- Sending or receiving emails with large attachments through Exchange Online.
- Observed behavior
- Large messages are delivered significantly later than expected, with message-trace values indicating delays in sender processing, EOP, or network transport.
Verify the total size of the email attachments you are trying to send and ensure you have access to the Microsoft 365 admin center to run a message trace.
Run a Message Trace to Identify Delay Stages
Use the Exchange admin center to identify whether the delay occurred within your organization, during network transport, or inside Exchange Online Protection.
Message-trace values help pinpoint the exact bottleneck. ExternalDelay reflects time spent outside the organization, while EopDelay reflects processing within Microsoft 365.
Log in to the Microsoft 365 admin center with administrator credentials and navigate to the Exchange admin center.
Go to Mail flow, select Message trace, and start a new trace by specifying the sender, recipient, and time range of the delayed email.
Review the trace results. Look specifically at 'ExternalDelay' and 'EopDelay' to determine if the large attachment caused throttling during security scans or external routing.
Share Large Files via Cloud Storage Instead of Email Attachments
Bypass Exchange Online email throttling by sending cloud links instead of attaching large files directly.
Avoid Email Attachment Limits with WPS Office Cloud Collaboration
Stop worrying about Exchange Online delays when sending large documents. WPS Office offers a free, lightweight alternative to Microsoft Office, featuring built-in cloud storage and instant link sharing so you can collaborate on large files seamlessly without attaching them to emails.
- 1. Download and Install WPS Office: Get WPS Office for free from the official website and install it on your device.
- 2. Save Your Document to WPS Cloud: Open your large document in WPS Office and save it directly to your free WPS Cloud storage space.
- 3. Share via Link: Click the 'Share' button in the top right corner to generate a secure link, and paste it into your email to avoid Exchange attachment delays.
Frequently Asked Questions
What does EopDelay mean in an Exchange message trace?
EopDelay represents the amount of time an email spends being processed by Exchange Online Protection (EOP). Large attachments often cause higher EopDelay because they require more time for malware and spam scanning.
Will upgrading my Microsoft 365 license fix email delays?
No. Upgrading your license or requesting a higher message size limit generally does not eliminate transport or throttling delays. Large files will still take time to process through network routing and security checks.
What is the maximum email size for Exchange Online?
By default, the maximum message size in Exchange Online is typically 35 MB, but administrators can increase it up to 150 MB. However, emails approaching this limit are highly susceptible to throttling and late delivery.
