When you upgrade to Joomla 3.5.1 Add user frontend and One click registration will stop working and give errors when the system tries to email. The error message you will see is: "0 - Invalid Adress".
Here is the manual fix for One Click Registration and Add User Frontend.
For O.C.R: open the file: /components/com_oneclickregistration/views/oneclickregistration/tmpl/default.php
For A.U.F: open the file: /components/com_adduserfrontend/views/adduserfrontend/tmpl/default.php
For both components:
Do a search for '$config->get' (without quotes)
You will find 4 results in total. You will find the following two lines of code twice:
$config->get( 'config.mailfrom' ),
$config->get( 'config.fromname' ) );
Remove the word config and the dot:
$config->get( 'mailfrom' ),
$config->get( 'fromname' ) );
New fixed version of AUF and OCR are released:http://www.joomlacy.com/extensions/components/one-click-registration/