How to enable SMTPUTF8 with Exim

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Introduction:
In today’s globalized digital landscape, email systems must be capable of handling internationalized content—including addresses with non-ASCII characters. This is where SMTPUTF8 comes into play.

SMTPUTF8 is an SMTP extension defined in RFC 6531 that allows email addresses and headers to contain UTF-8 encoded characters. This enables the use of accented letters, non-Latin scripts, and other Unicode characters, making email more accessible and inclusive across different languages and regions.

If our clients are attempting to use SMTPUTF8 while your server does not have it enabled, you may encounter errors similar to the following in the /var/log/exim_mainlog:

2025-12-15 15:26:00.383 [3539516] SMTP syntax error in "MAIL FROM: <test@example.tld> SMTPUTF8

" H=(hostname) [xxx.xxx.xxx.xxx]:55653 I=[xxx.xxx.xxx.xxx]:465 SMTPUTF8 used when not advertised

This error occurs because the client requests SMTPUTF8 support that the server has not advertised, causing the server to reject the message.

To resolve this issue and ensure smooth communication with modern mail clients, you need to enable specific settings in Exim to support SMTPUTF8. The procedure below outlines how to do this through WHM (Web Host Manager).

Procedure to Enable SMTPUTF8 in Exim

  1. Log in to WHM as the root user
    • Access your WHM (Web Host Manager) using root credentials.
  2. Navigate to Exim Configuration Manager
    • Go to: Home » Service Configuration » Exim Configuration Manager
  3. Access the Advanced Editor
    • Click on the Advanced Editor tab.
  4. Modify Configuration Settings
    • Locate the Add Additional Configuration Setting button and click on it.
    • Add the following configuration lines:

      allow_utf8_domains = true

      smtputf8_advertise_hosts = *
  5. Save Changes
    • Scroll to the bottom of the page and click the Save button to apply the changes.

After completing these steps, Exim should properly support SMTPUTF8, allowing email clients to send and receive UTF-8 encoded emails without issues.

Conclusion:
Enabling SMTPUTF8 in Exim ensures better email compatibility with modern international character sets. Following the above steps will help prevent syntax errors and improve overall email handling on your server.

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