Removed mention of Let's Encrypt for S/MIME certificates

Author: Thomas Berends

mailer.rst

@@ -561,7 +561,7 @@ encrypt a signed message and/or to sign an encrypted message.
 Before signing/encrypting messages, make sure to have:
 
 * The `OpenSSL PHP extension`_ properly installed and configured;
-* A valid security certificate (you can get it for free from `Let's Encrypt project`_).
+* A valid S/MIME security certificate.
 
 Signing Messages
 ~~~~~~~~~~~~~~~~