-/* If there's a colon in the key, temporarily replace it with
-a zero to terminate the string there. */
+/* If there's a colon in the key, there are two possibilities:
+
+(1) This is a key for a host, ip address, and possibly port, in the format
+
+ hostname:ip+port
+
+ In this case, we temporarily replace the colon with a zero, to terminate
+ the string after the host name.
+
+(2) This is a key for a pipe, file, or autoreply delivery, in the format
+
+ pipe-or-file-or-auto:x@y
+
+ where x@y is the original address that provoked the delivery. The pipe or
+ file or auto will start with | or / or >, whereas a host name will start
+ with a letter or a digit. In this case we want to use the original address
+ to search for a retry rule. */