We have probably the same problem. At least a few times users received not their subscription - daily digest and weekly summaries as well.
In logs I found an error which shows up from time to time (e.g. yesterday everything seems to be fine but today emails were mixed):
PHP Parse error: syntax error, unexpected T_STRING in /wp-content/plugins/buddypress-group-email-subscription/bp-activity-subscription-digest.php(463) : runtime-created function on line 207
It's multiplicated many times (I guess for many emails) but sometimes there are 1000 errors for 5000 emails, sometimes more, the other day there is no errors and emails seem to be sent correctly. I'm not 100% sure that this error is connected with this issue but I can't find any other clue.
the code looks like:
add_action( 'phpmailer_init', create_function( '$phpmailer', '
$phpmailer->Body = "<html><body>' . addslashes( $message ) . '</body></html>";
$phpmailer->AltBody = "' . $message_plaintext . '";
' ) ); <-- here is the line with error
This is multisite instalation so we use many plugins (e.g. WPML, Gravity Forms, many security plugins but no cache plugins). All plugins are up to date.
BuddyPress Group Email Subscription 3.3.1 WP 3.5, BP 1.6.2 . PHP 5.3.8.
Our network has 500+ groups and 5000+ users and many groups are private so the problem is critical. Any help will be very appreciated. Thanks in advance.