To send a reminder or follow up email to anyone who didn't open your original e-blast you should create a segment to select contacts who didn't open your email, then resend your email to these contacts. Related article: Open tracking in emails.
When you resend the message, anyone who have unsubscribed since the original email was sent will automatically be excluded from the resend.
Step 1
Go to the segments page and create a new segment. You can call it "Didn't open MESSAGE NAME". Click "Save".
Step 2
Add a condition for "Campaign status" = didn't open CAMPAIGN NAME
Your segment will look like this:
Step 3
Go back to the messages page and duplicate your original message. You can call it "Resend of MESSAGE NAME". The resend should use a slightly different subject line: maybe the original didn’t pique their interest and the updated version will. It's also a good idea to resend at a different time than the original send time.
Step 4
Select the recipients for the new message using the "Segment" you created in Step 2 above.
Step 5
Send yourself a test and after making sure all looks good, send away!
Related article: Re-engaging inactive contacts.
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.
This applies only if you are on a pay-as-you-go plan. The monthly contact-based subscription includes unlimited email sends.