paypal ipn problems

Hey,
The plugin seemts to have problems with paypal ipn messages. I have read previous posts about it and did necessarry changes but still cant take ipn messages when plugin is active, had to disable it.
Also when i disabled it, a ipn message came thru which was sent earlier on. Not sure if ipn repeats itself for a period, but the payment was done before I disabled the plugin, and came thru after I enabled it (talking about a 15 minute gap)
What should be taken care of in order to have this work with paypal ?

Thanks

https://ww.wp.xz.cn/plugins/stop-spammer-registrations-plugin/