auto retrieve keys from PKA. Thsi allows to specify an email address

so that gpg can get the key from DNS.  This helps with opportunistic
encryption.  No integration with the trust modell yet.

g10/misc.c

@@ -1088,6 +1088,20 @@ has_invalid_email_chars (const char *s)
 }
 
 
+/* Check whether NAME represents a valid mailbox according to
+   RFC822. Returns true if so. */
+int
+is_valid_mailbox (const char *name)
+{
+  return !( !name
+            || !*name
+            || has_invalid_email_chars (name)
+            || string_count_chr (name,'@') != 1
+            || *name == '@'
+            || name[strlen(name)-1] == '@'
+            || name[strlen(name)-1] == '.'
+            || strstr (name, "..") );
+}
 
 
 /* This is a helper function to load a Windows function from either of