spec: TLS certificates: avoid MD5

Make it clearer in the spec, where talking about certificates, that MD5
in certs is a really Quite Bad idea.

diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt
index 614259a5d5f46209788cad4b6bfdf2ae2d89900e..4b9f53ed14073603c012951509783ef90b0abbae 100644 (file)
--- a/doc/doc-docbook/spec.xfpt
+++ b/doc/doc-docbook/spec.xfpt
@@ -26057,6 +26057,12 @@ validation to succeed, of course, but if it's not preinstalled, sending the
 root certificate along with the rest makes it available for the user to
 install if the receiving end is a client MUA that can interact with a user.
 
+Note that certificates using MD5 are unlikely to work on today's Internet;
+even if your libraries allow loading them for use in Exim when acting as a
+server, increasingly clients will not accept such certificates.  The error
+diagnostics in such a case can be frustratingly vague.
+
+
 
 .section "Self-signed certificates" "SECID187"
 .cindex "certificate" "self-signed"