You can send Open Inmail messages to leads on LinkedIn who are not in your network but have a Sales Navigator account and an Open Profile.
Open Profile means that this person accepts messages from outside their network, therefore you can add them to your Open InMail campaigns and message up to 800 people per month via such campaign.
Since LinkedIn doesn't have a filter for Open Profiles, you can find these people using Prospect Labs Assignment filter amongst your search results in Search.
To find leads who have Open Profiles and can accept Open InMails, you can use the Assignment filter in Prospect Labs:
Create a usual search on LinkedIn.com or Sales Navigator
Import your search results to Prospect Labs searches
Once the search is complete, click on the eye icon to view the results:
Click on Filters, select Assignment filter and choose Show Open Inmail:
You will have a list of people from your reach results who can accept Open InMail messages
Click on Actions, create a tag called Open InMail, then tick and tag all this list of people:
This way you will be able to make sure that all Open Profile people are in the right campaign.
Go back to Filter, click on Filter on tags:
Filter your search now by Open InMail tag:
a) Scroll to the bottom of the page, select to view 100 people per page and b) tick all people on this page at the top:
Amongst the filters, click on Actions and select "Assign to a campaign":
Select your Open InMail campaign and click "Add Filtered Contacts To Campaign":
Check your Open InMail campaign to make sure only tagged people are added to this campaign.