I need more complex condition for e-mail notification than simple send same email to the same recipients when the same alarm is generated.
The easiest way to achieve this is to use a NXSL script as the Event Processing Policy action. The only problem is that there is no possibility to send an e-mail from a NSXL script therefore I've added the NSXL function SendMail which allows this.

Please add the changes to the original repositories.
Code: https://github.com/netxms/netxms/pull/39
Documentation: https://github.com/netxms/nxsl-doc/pull/5