To send a reminder or follow up email to anyone who didn't open your original e-blast you can create a filter to select contacts who didn't open your email, then send the reminder to the contacts matched by this filter.
Go to the filters page and create a new filter. You can call it "Didn't open MESSAGE NAME". Click "Save".
Add a new condition to the filter by selecting "Campaign status"
Select "Didn't open" as the condition and select your original campaign in the "value" field.
Your filter will look something like this:
Go back to the messages page and duplicate your original email message you want to re-send. You can call it "Re-send of MESSAGE NAME".
In the "To" field use the "Filter" option and select the filter you created in Steps 1 - 3 above.
How opens are tracked
Opens are tracked using a tracking pixel, which is a small, one pixel, transparent image that loads when emails are opened. Since it's an image, if contacts block images in emails, the tracking pixel cannot load and opens aren't counted.
Non-opens are defined as contacts where the tracking pixel didn't load in their emails.
Email credits used
Emails in the "resend" campaign use credits the same way other email messages do; one email sent to one contact = 1 credit.