com.sun.mail.util.MailLogger
is part of JavaMail API. It is already included in EE environment (that's why you can use it on your live server), but it is not included in SE environment.
The JavaMail API is available as an optional package for use with Java SE platform and is also included in the Java EE platform.
99% that you run your tests in SE environment which means what you have to bother about adding it manually to your classpath when running tests.
If you're using maven add the following dependency (you might want to change version):
<dependency>
<groupId>com.sun.mail</groupId>
<artifactId>javax.mail</artifactId>
<version>1.6.0</version>
</dependency>