How can I send a mailing to everyone who hasn't responded yet to an initial mailing?

You'll need to make a temporary group or two to do it.

You'll copy the email from before and select the group that you had before (or select the recipients of the prior email).

You'll exclude the people in the groups that you make. Make the group(s) of the people who opened or already RSVP'd to your event first

Here's how to make the groups-

Step 1 - People that have opened the email

go to Search>Advanced Search

Select Mailings accordion.

Select your mailing

click the button for opened so that you get those who opened it.

Click Search take the option to add these people to a temporary regular group.

Step 2 - People that have RSVP'd

go to Search>Advanced Search

Select the Events Accordion

Select the event you are tracking

Select the appropriate participant status, if needed.

Click Search take the option to add these people to the group that you created above.

Step 3 - Sending the next Email

Now, when you prepare your next email, exclude the group you created from the mailing.

Optionally, these people can be put in smart groups, but you'll need to use two different ones.

Email and Postal Mailings