send fails on copy of site

I have CF7 installed and working perfectly on my production site.
I made a copy of that site to another site for testing purposes. The copy was made by
a) backing up the database and restoring the backup to the test db
b) copying the entire file structure to the test site
c) adapting config.php as required
d) changing all the URLs in all tables from the prod URL to the test URL

Now, when I complete a form and click on Send in the test site, the send fails (I get the plugin’s failure message).

Reinstalling the plugin changes nothing.

So what have I forgotten to do?
Thanks!