How to email a course section in moodle.
The simplest way to email a section is to use the section course site and email the participants on that site. Even if you don't use the site for anything else and don't make it visible to the students, you as an instructor can still use its mailing ability.
There is no easy way to email multiple (but not all) sections at the same time. You either have to repeat the mailing in each of the section sites or select the students you want to email in the umbrella course site. If you do this alot you may want to create a group to send email to. That involves selecting all the students for the group and then using the With selected user pulldown to create a group. For this to work, the Groups setting for the course must be set to visible or separate.
Note that individual sections are different than co-requisite courses such as labs connected with science courses. Mailing members of co-requisites can be done from from the umbrella courses.