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.
The additional opens resends generate typically far offsets contacts you lose due to unsubscribes in resend campaigns
When you resend the message, anyone who have unsubscribed since the original email was sent will automatically be excluded from the resend.
Go to the segments page and create a new segment. You can call it "Didn't open MESSAGE NAME". Click "Save".
Add a condition for "Campaign status" = didn't open CAMPAIGN NAME
Your segment will look like this:
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.
Select the "Segment" you created in Step 2 above when you select the list of recipients for your message.
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.