In WordPress Multi-Site setups where a blog has no users of the role "administrator" assigned to it, Subscribe2 (as of version 8.2) will always send e-mail from the default e-mail account specified for the blog, and you will be unable to select who e-mails should be sent from on the settings page, seeing instead this warning:
Warning: array_merge() [function.array-merge]: Argument #2 is not an array in /...../ 431
Line 431 of class-s2-admin.php reads:
$admins = array_merge($author, $admins);
There should be a sanity-check (has it been intialized to an array?) on $admins, as it may be empty in WPMS setups.
A solution to this was to explicitly add my network admin account as an administrator on the blog in question.