Last updated: 2026-02-11

Exim TLS/SSL Configuration Guide

This guide provides recommended TLS/SSL settings for Exim, a highly configurable mail transfer agent. Exim is the default MTA on Debian and Ubuntu systems. Unlike most MTAs, Exim can be built against either OpenSSL or GnuTLS, and the syntax for cipher and protocol configuration differs between the two. This guide covers both variants.

Prerequisites

Determine Your TLS Library

Exim's TLS behavior depends on which library it was compiled against. Check with:

exim -bV | grep TLS

You will see either OpenSSL or GnuTLS in the output. This determines which cipher and protocol syntax to use throughout this guide.

Distribution defaults: Debian and Ubuntu build Exim with GnuTLS. RHEL and derivatives build Exim with OpenSSL.

Certificate Configuration

tls_certificate = /etc/exim/ssl/fullchain.pem
tls_privatekey = /etc/exim/ssl/privkey.pem

Path note: RHEL-based systems use /etc/exim/. Debian and Ubuntu use /etc/exim4/. On Debian/Ubuntu with split configuration, place TLS settings in /etc/exim4/conf.d/main/01_exim4-config_tls or use dpkg-reconfigure exim4-config and edit the unsplit config file.

Server TLS Configuration (Incoming SMTP)

These settings control how Exim handles incoming SMTP connections on port 25.

Advertise STARTTLS

tls_advertise_hosts = *

This advertises STARTTLS to all connecting hosts. Exim does not have a single "security level" setting like Postfix; instead, you control mandatory TLS through ACLs (see Submission Port).

Protocol Versions

OpenSSL builds:

openssl_options = +no_sslv2 +no_sslv3 +no_tlsv1 +no_tlsv1_1

GnuTLS builds:

Protocol versions are controlled through the GnuTLS priority string in tls_require_ciphers (see Cipher Suites below). The -VERS-ALL:+VERS-TLS1.2:+VERS-TLS1.3 component restricts to TLS 1.2 and 1.3.

The +no_tlsv1 and +no_tlsv1_1 options require Exim 4.91+. Earlier versions only support +no_sslv2 and +no_sslv3.

Cipher Suites

OpenSSL builds:

tls_require_ciphers = ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305

GnuTLS builds:

tls_require_ciphers = SECURE256:-VERS-ALL:+VERS-TLS1.2:+VERS-TLS1.3

The GnuTLS priority string controls both cipher suites and protocol versions in a single directive. SECURE256 selects strong cipher suites with 256-bit security level (it also includes 128-bit AES-GCM). The -VERS-ALL:+VERS-TLS1.2:+VERS-TLS1.3 portion restricts to TLS 1.2 and 1.3.

Important: tls_require_ciphers in the main configuration section applies to incoming (server-side) connections. The same directive used inside a transport block applies to outgoing (client-side) connections.

ECDH Curve Selection

tls_eccurve = P-256:P-384

This selects the elliptic curves for ECDHE key exchange. Requires Exim 4.93+.

Logging

log_selector = +tls_cipher +tls_peerdn +tls_sni

This adds TLS cipher, peer certificate DN, and SNI information to the main log.

Client TLS Configuration (Outgoing SMTP)

These settings control how Exim connects to other mail servers when delivering outbound mail. They are configured in the remote_smtp transport block.

Enable Opportunistic TLS

In the remote_smtp transport:

remote_smtp:
    driver = smtp
    hosts_try_tls = *

Certificate Verification

remote_smtp:
    driver = smtp
    hosts_try_tls = *
    tls_try_verify_hosts = *
    tls_verify_certificates = /etc/ssl/certs/ca-certificates.crt

Client Cipher Settings

You can set client-side cipher preferences in the transport block:

OpenSSL builds:

remote_smtp:
    driver = smtp
    hosts_try_tls = *
    tls_require_ciphers = ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305

GnuTLS builds:

remote_smtp:
    driver = smtp
    hosts_try_tls = *
    tls_require_ciphers = SECURE256:-VERS-ALL:+VERS-TLS1.2:+VERS-TLS1.3

DANE Support

DANE (DNS-Based Authentication of Named Entities) uses TLSA DNS records to verify server certificates without relying on certificate authorities.

Enable DANE

In the remote_smtp transport:

remote_smtp:
    driver = smtp
    hosts_try_dane = *
    hosts_try_tls = *
    tls_try_verify_hosts = *
    tls_verify_certificates = /etc/ssl/certs/ca-certificates.crt
    dane_require_tls_ciphers = ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305

DNSSEC Requirement

DANE requires a DNSSEC-validating DNS resolver. Exim must also be compiled with SUPPORT_DANE=yes (enabled by default in Exim 4.91+ on most distributions).

Submission Port (587)

For authenticated email submission from mail clients, configure Exim to listen on port 587 with mandatory TLS. Add to the main configuration:

daemon_smtp_ports = 25 : 587
tls_on_connect_ports = 465

Port 465 (SMTPS) uses implicit TLS, where the connection is encrypted from the start. Ports 25 and 587 use STARTTLS.

Require TLS on Port 587

In the acl_check_rcpt ACL (or your RCPT ACL), add a rule early in the ACL to deny unauthenticated or unencrypted submission:

deny
    condition = ${if eq{$interface_port}{587}}
    !encrypted = *
    message = TLS required on submission port

deny
    condition = ${if eq{$interface_port}{587}}
    !authenticated = *
    message = Authentication required on submission port

These rules ensure that connections on port 587 must use TLS encryption and SMTP authentication before mail is accepted.

Version Notes

Several features depend on specific Exim versions. Enterprise distributions often ship older versions:

Distribution versions:

SLES 15 does not include Exim in its base repositories.

Complete Configuration

The following is a complete TLS configuration for an OpenSSL-based Exim build. GnuTLS alternatives are noted in comments.

Main Configuration

# Certificate files
tls_certificate = /etc/exim/ssl/fullchain.pem
tls_privatekey = /etc/exim/ssl/privkey.pem

# Advertise STARTTLS to all hosts
tls_advertise_hosts = *

# Listen on SMTP, submission, and SMTPS ports
daemon_smtp_ports = 25 : 587
tls_on_connect_ports = 465

# Protocol versions (OpenSSL)
# On GnuTLS builds, protocol versions are controlled in tls_require_ciphers below
openssl_options = +no_sslv2 +no_sslv3 +no_tlsv1 +no_tlsv1_1

# Server cipher suites (OpenSSL)
# On GnuTLS builds, use instead:
#   tls_require_ciphers = SECURE256:-VERS-ALL:+VERS-TLS1.2:+VERS-TLS1.3
tls_require_ciphers = ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305

# ECDH curves (requires Exim 4.93+; omit on older versions)
tls_eccurve = P-256:P-384

# TLS logging
log_selector = +tls_cipher +tls_peerdn +tls_sni

ACL Configuration

In your acl_check_rcpt ACL:

# Require TLS on submission port 587
deny
    condition = ${if eq{$interface_port}{587}}
    !encrypted = *
    message = TLS required on submission port

# Require authentication on submission port 587
deny
    condition = ${if eq{$interface_port}{587}}
    !authenticated = *
    message = Authentication required on submission port

Transport Configuration

remote_smtp:
    driver = smtp
    hosts_try_tls = *
    hosts_try_dane = *
    tls_try_verify_hosts = *
    tls_verify_certificates = /etc/ssl/certs/ca-certificates.crt
    # On RHEL, use: /etc/pki/tls/certs/ca-bundle.crt
    # Client cipher suites (OpenSSL)
    # On GnuTLS builds, use instead:
    #   tls_require_ciphers = SECURE256:-VERS-ALL:+VERS-TLS1.2:+VERS-TLS1.3
    tls_require_ciphers = ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305
    dane_require_tls_ciphers = ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305

Verification

Check the Exim build and TLS library:

exim -bV | grep TLS

Test inbound STARTTLS on port 25:

openssl s_client -connect mail.example.com:25 -starttls smtp

Test implicit TLS on port 465:

openssl s_client -connect mail.example.com:465

Debug TLS for a specific address:

exim -d+tls -bt user@example.com

Check the main log for TLS connections:

grep tls_cipher /var/log/exim/mainlog

Test your MX server's TLS configuration at https://www.checktls.com/ or https://ssl-tools.net/mailservers.