I've created a D7 implementation of the SMTP stuff for Devel.
It still probably needs a little love in the documentation department, but here's the deal:
variable_set('mail_system', array('default-system', 'DevelMailLog'));
Enables it. Unless you set anything else mails are saved to files/mails/$to-$subject-$datetime.mail.txt
You can change the directory with
variable_set('devel_debug_mail_directory', file_directory_path() . '/mails');
Or the file format
Currently, only those three tokens are supported, and NOT through token module.
I also added a test which is *almost* a pure unit test, but because of the variable_get's couldn't use the DrupalUnitTest base class ;(