So, to get the activation URL I used a BeanShell PostProcessor. If you use BeanShell you need to place bsh-2.0b4.jar in the JMeter lib directory.
The URL I needed to check was from the following format: https://www.[domain].be/Action.do?URL=[Activation Code]"
I only needed the Activation Code. This is the BeanShell PostProcessor code I used:

import org.apache.jmeter.samplers.SampleResult;

counter = 1;
temp=null;
newVal = "INVALID";

//get all mails
results = prev.getSubResults();
	for(SampleResult result: results) {
		mail = result.getResponseDataAsString();
                //check sender and subject
		if(mail.contains("From: [Sender]") && mail.contains("Subject: [Subject]")) {
                        //get URL
			temp = mail.split("URL=");
			temp = temp[1].split(”\”>”);
			newVal = temp[0];
		}
        //put url in JMeter variable
	vars.put("activationCode", newVal);
	counter++;
	}

The Activation code is now in the JMeter variable: $[activationCode] which can be accessed from Samplers, Pre & Post-Processors,...
To access this variable, you need to define it in JMeters User Defined Variables first. Otherwise JMeter doesn't know about it existence.

I used a split to get the activation code out of the URL, you could also use a regex which would be something like this: .*URL=([^"]*).*