You can use the com.sun.mail classes that are described in the javadocs. Unlike the standard javax.mail classes, which we try very hard to keep stable and compatible, there's a small chance that the com.sun.mail classes will evolve incompatibly in future releases. We understand that many people use these classes, so we would not make such changes lightly, but if necessary to improve the JavaMail implementation or to support new standard API classes, they might change.
Note that there are also lots of com.sun.mail classes that are not described in the javadocs. Use of those is much higher risk; they're much more likely to change incompatibly in even minor releases.