This document explains how to get Email policies to work in conjunction with PX , that is Policy Xpress, policies.
Email policies as well as Policy Xpress policies have priority fields. One may imagine the priorities of the two are same. But they are not. The email policies are run by a separate handler thread than the PX policies and therefore relying on the priority field will not make it work.
Any Identity Manager version.
There isn't a way to synchronize PX policies and Email policies. They are independently react or respond to the IDM events. So if you have them both trigger on Task Completion (for example) then there isn't a way to control who goes first. PX policies have a priority mechanism that applies to policies within PX but it does not apply to Email policies.
IDM is an asynchronous application and therefore anything that isn't synchronized within IDM itself (such as the PX policies themselves according to their priorities) is asynchronous. This does explain the intermittent behavior you are experiencing.
You should trigger the email from within your PX policy itself. In PX, under Action, there is a category for Mail. You can build your action rule to trigger your email from there as the last action rule of your PX policy (after all else was done).