Introduction

Sometimes, PHP mail scripts may fail while sending emails through Exim due to TLS-related issues with remote SMTP servers. This issue can generate transport errors in the mail logs.

Prerequisites

  • Root or SSH access to the server
  • Access to the Exim configuration file
  • Basic knowledge of mail server configuration

Implementation

Problem

The following error may appear while sending emails using PHP scripts:

T=remote_smtp defer (-1): smtp transport process returned non-zero status 0x000b:
terminated by signal 11

Resolution

Edit the Exim configuration file:

/etc/exim.conf

Locate the following section:

remote_smtp:
driver = smtp

Change it to:

remote_smtp:
driver = smtp
hosts_avoid_tls=*

Save the configuration file and restart Exim.

Explanation

If Exim is built with TLS support and the remote mail server advertises TLS, the SMTP transport will automatically attempt to establish a TLS session.

This behaviour can be prevented by using the option:

hosts_avoid_tls=*

This tells Exim not to use TLS for SMTP connections to remote hosts.

Conclusion

By modifying the Exim SMTP transport configuration and disabling TLS for remote SMTP connections, you can resolve PHP mail sending issues related to signal 11 and SMTP transport failures.

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