When geronimo-mail in on the classpath (perhaps in addition to the original SUN mail implementation), the POP3 polling connector fails with an NPE within the Geronimo code. This NPE originates in the Mule code from RetrieveMessageReceiver.java:130, where a new instance of the MimeMessage is created. In the geronimo-mail implementation, the dh-field is used to perform a deep copy of the message contents. However, this field is null.
Although this isn't necessarily a Mule isse (as mule ships with the correct dependency), it's not an easy thing to find for development. Further, it's a bit odd that Mule fails when using the different implementations of the same spec. During development, this issue can easily come up when using e.g. Maven.
The fix may be as simple as just mentioning this in the docs, with a big warning sign .