The user configures email like this:

<string name="jbpm.mail.smtp.host" value="mail.xxx.int" />
<bean name="jbpm.mail.address.resolver" class="org.jbpm.identity.mail.IdentityAddressResolver" singleton="true" />
<string name="jbpm.mail.from.address" value="xxxsoa@yyynetwork.it" />
<string name="jbpm.mail.user" value="xxxsoa@yyynetwork.it" />
<string name="jbpm.mail.pass" value="xxxxxxx" />
<string name="jbpm.mail.port" value="25" />
<string name="jbpm.mail.smtp.auth" value="true" />
<string name="jbpm.mail.debug" value="false" />

Then they use an Email node in their process.

For recipients on the domain 'yyynetwork.it', there is no problem. (This domain doesn't require authentication.) But if the recipient is on some other domain, they get this Exception: "Caused by: com.sun.mail.smtp.SMTPAddressFailedException: 554 5.7.1 <some.person@someplace.com>: Relay access denied"

It is suspected that this is similar to an issue addressed in jBPM-3032. Here, the suggested fix is to use javax.mail.PasswordAuthentication. That may be what's required here, as well.