Understanding and Configuring Maximum Email Size in Symantec Email Security.Cloud
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Understanding and Configuring Maximum Email Size in Symantec Email Security.Cloud

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Article ID: 406156

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Updated On:

Products

Email Security.cloud

Issue/Introduction

Symantec Email Security.Cloud enforces email size limits to protect infrastructure performance and reduce delivery failures. However, many administrators encounter unexpected email rejections due to the way email messages grow in size during transit — a process called email expansion.

This article explains how Symantec Email Security.Cloud calculates maximum message size, the effect of base64 encoding, and how to correctly configure limits in the ClientNet portal to align with your organisation’s needs.

Environment

Symantec Email Security.Cloud

Resolution

Why Emails Appear Smaller but Get Rejected
Email messages transmitted over the Internet are subject to expansion. This happens mainly due to MIME encoding, especially base64, which is used for sending binary content like attachments or rich-text formatting.

Key Concepts:

  • Base64 Encoding: Converts binary files (like images or PDFs) to ASCII text. This process increases the size of the data by approximately 33%.
  • Email Expansion: The encoded message sent to Symantec can be 30%–40% larger than what users see in their mail client (e.g., Outlook or Gmail).
  • Impact: A user may attach a 20 MB file and see no issue locally, but Symantec may reject it if the total encoded size exceeds the configured message limit.

 

ClientNet Settings vs. Actual Message Size
Symantec allows email administrators to set the maximum email size (in megabytes) via the ClientNet portal. However, this limit applies to the encoded size of the message — not what users typically see in their email client.

Here’s a reference table:

 

ClientNet Configured Limit (MB) Approx. Max Size Seen by User (MB)
5.00 MB 3.50 MB
10.00 MB 7.10 MB
20.00 MB 14.30 MB
30.00 MB 21.50 MB
40.00 MB 28.70 MB
50.00 MB 35.80 MB
52.00 MB (Maximum allowed) ~37.30 MB

 
Note: These are approximate values. Real-world results may vary depending on the message content and encoding overhead.

 

Recommendations for Administrators:


Step 1: Determine User Requirements

  • Estimate the maximum message size (including attachments) users are expected to send. For example, if users commonly send 15 MB attachments, you should account for expansion.

Step 2: Use the Size Conversion Table

  • Refer to the table above and select a ClientNet limit that safely accommodates expanded message size. In the example of 15 MB user messages, a 20–25 MB setting is advisable.

Step 3: Update ClientNet Settings

  • Log in to the ClientNet Portal.
  • Navigate to: > Services > Email Services > Message Size
  • Modify the "Maximum Email Size" field.
  • Save and apply changes.

 

Additional Information

Troubleshooting Email Size Rejections

If users report delivery failures due to message size:

  1. Check Symantec Message Track and Trace logs.

  2. Look for errors indicating that the message exceeded the size restriction.

  3. Compare the client-visible size to the encoded size threshold.

  4. Adjust the ClientNet setting if necessary.

Email expansion is a critical factor when configuring message size limits in Symantec Email Security.Cloud. By understanding how base64 encoding affects transmission size and aligning ClientNet settings with actual usage, administrators can minimize message rejections and improve email delivery reliability.

References

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