diff -NurBbp a/doc/spec.txt exim-4.80/doc/spec.txt
--- a/doc/spec.txt	2012-05-31 11:35:23.000000000 +0200
+++ exim-4.80/doc/spec.txt	2012-06-08 13:08:19.000000000 +0200
@@ -21221,6 +21221,17 @@ This option specifies a certificate revo
 the name of a file that contains a CRL in PEM format.
 
 +--------------+---------+-------------+--------------+
+|tls_dh_min_bits|Use: smtp|Type: integer|Default: 1024|
++-----------------------------------------------------+
+
+When establishing a TLS session, if a ciphersuite which uses Diffie-Hellman key
+agreement is negotiated, the server will provide a large prime number for use.
+This option establishes the minimum acceptable size of that number. If the
+parameter offered by the server is too small, then the TLS handshake will fail.
+
+Only supported when using GnuTLS.
+
++--------------+---------+-------------+--------------+
 |tls_privatekey|Use: smtp|Type: string*|Default: unset|
 +--------------+---------+-------------+--------------+
 
@@ -23630,6 +23641,11 @@ There are some differences in usage when
   * The tls_require_ciphers options operate differently, as described in the
     sections 41.4 and 41.5.
 
+  * The tls_dh_min_bits SMTP transport option is only honoured by GnuTLS. When
+    using OpenSSL, this option is ignored. (If an API is found to let OpenSSL
+    be configured in this way, let the Exim Maintainers know and we'll likely
+    use it).
+
   * Some other recently added features may only be available in one or the
     other. This should be documented with the feature. If the documentation
     does not explicitly state that the feature is infeasible in the other TLS
