Integration

Hi All,
i have a process running on bpel 10.1.3,
this process sends email notifications to a number of users at different stages.
one particular user fails to receive the notification, i checked the log and i found the following error:

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class javax.mail.SendFailedException: 550 5.1.1 User unknown

07/04/20 08:36:14 javax.mail.SendFailedException: Invalid Addresses;
nested exception is:
class javax.mail.SendFailedException: 550 5.1.1 User unknown

07/04/20 08:36:14 at com.sun.mail.smtp.SMTPTransport.rcptTo(SMTPTransport.java:926)
07/04/20 08:36:14 at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:389)
07/04/20 08:36:14 at oracle.tip.pc.services.notification.email.EmailDriver.sendMessage(EmailDriver.java:216)
07/04/20 08:36:14 at oracle.tip.pc.services.notification.email.EmailDriver.send(EmailDriver.java:186)
07/04/20 08:36:14 at oracle.tip.pc.services.notification.DefaultNotificationServiceImpl.sendEmailNotification(DefaultNotificationServiceImpl.java:251)
07/04/20 08:36:14 at oracle.tip.pc.services.notification.NotificationServiceImpl.sendEmailNotification(NotificationServiceImpl.java:271)
07/04/20 08:36:14 at oracle.bpel.services.notification.queue.sender.MDBConsumer.deliverNotification(MDBConsumer.java:256)
07/04/20 08:36:14 at oracle.bpel.services.notification.queue.sender.MDBConsumer.onMessage(MDBConsumer.java:137)
07/04/20 08:36:14 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
07/04/20 08:36:14 at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
07/04/20 08:36:14 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
07/04/20 08:36:14 at java.lang.reflect.Method.invoke(Method.java:585)
07/04/20 08:36:14 at com.evermind.server.ejb.interceptor.joinpoint.EJBJoinPointImpl.invoke(EJBJoinPointImpl.java:35)
07/04/20 08:36:14 at com.evermind.server.ejb.interceptor.InvocationContextImpl.proceed(InvocationContextImpl.java:119)
07/04/20 08:36:14 at com.evermind.server.ejb.interceptor.system.DMSInterceptor.invoke(DMSInterceptor.java:52)
07/04/20 08:36:14 at com.evermind.server.ejb.interceptor.InvocationContextImpl.proceed(InvocationContextImpl.java:119)
07/04/20 08:36:14 at com.evermind.server.ejb.interceptor.system.SetContextActionInterceptor.invoke(SetContextActionInterceptor.java:44)
07/04/20 08:36:14 at com.evermind.server.ejb.interceptor.InvocationContextImpl.proceed(InvocationContextImpl.java:119)
07/04/20 08:36:14 at com.evermind.server.ejb.InvocationContextPool.invoke(InvocationContextPool.java:55)
07/04/20 08:36:14 at com.evermind.server.ejb.MessageDrivenConsumer.onMessage(MessageDrivenConsumer.java:347)
07/04/20 08:36:14 at com.evermind.server.ejb.MessageDrivenConsumer.processMessages(MessageDrivenConsumer.java:233)
07/04/20 08:36:14 at com.evermind.server.ejb.MessageDrivenConsumer.run(MessageDrivenConsumer.java:169)
07/04/20 08:36:14 at com.evermind.util.ReleasableResourcePooledExecutor$MyWorker.run(ReleasableResourcePooledExecutor.java:298)
07/04/20 08:36:14 at java.lang.Thread.run(Thread.java:595)
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the user name and the email of the person are correct, does anybody have any idea of what could be the problem? thanks