Use the "If Sent Email" condition to send to specific contacts, based on whether a previous email: 

• Was not sent to them;
• Or Was sent to them.

Example use: Was not sent

You would like to send out a "Data Discovery" email, but don't want to send it to any contacts that have received it recently.

In this example, set the If Sent Email condition to Was not sent, and select the email as Data Discovery. You can optionally set a timeframe, so that you can be precise about the timing of the previous email, e.g.: Within the last 30 days. 

In the following Send Email module, select the same email, e.g.: Data Discovery.

With these settings, all contacts who received the Data Discovery email more than 30 days ago (or never), will be sent the email. Contacts that have received the Data Discovery email within the last 30 days will be excluded.*

* In this example, Choose recipients is set to Send to ALL Contacts, as the trigger acts as a contact filter.
You could alternatively select certain tags and/or audiences, to further filter your reach.

Example use case: Was sent

You want to send a follow up on your "Investor" email (perhaps from a different campaign, or even multiple other campaigns), and only want to send to contacts that were sent this specific email.

* You could alternatively combine the emails into one drip campaign.

In this example, you would set the condition to Was sent, and select the email you want to follow up on, e.g.: "Investor email #1".
In the following Send Email module, select "Investor Email #2" 
You can optionally set a timeframe, so that you can be precise about the timing of the previous email, e.g.: (Was sent) Within the last 14 days. 

With these settings, all contacts who received Investor email #1 within the last 14 days will be sent Investor email #2. All other contacts will be excluded.*

* In this example, Choose recipients is set to Send to ALL Contacts, as the trigger acts as a contact filter.
You could alternatively select certain tags and/or audiences, to further filter your reach.