"If you are using EJB then you can use a MDB (Message Driven Bean) to receive email. JBoss provides a JCA adaptor — mail-ra.rar — but the version distributed with JBoss AS has a number of limitations (and isn't bundled in some versions) therefore we recommend using the mail-ra.rar distributed with Seam (it's in the extras/ directory in the Seam bundle). mail-ra.rar should be placed in $JBOSS_HOME/server/default/deploy; if the version of JBoss AS you use already has this file, replace it."
To say if using JBoss AS 5.x and above the mail-ra.rar changes are merged into JBoss AS already. On AS 4.x it is still tech preview. The changes being fixes to the limitations that the docs originally noted, there should no longer be these limitations in JBoss AS 5.x but they are still in AS 4.x
Pete has said that his patches for the mail-ra.rar were all accepted into JBAS so these changes should be in the JBAS 5.x, thus the mail-ra.rar is up to par.