Sendmail error unable to relay

We use the 2.4.1 #55 standalone. When I send mail to users outside our own domain, I get the following JAVA errors:
---------------------------------------------------
INFO: Initializing WorkflowStore: com.opensymphony.workflow.spi.ofbiz.OfbizWorkflowStore
javax.mail.SendFailedException: Sending failed;
nested exception is:
javax.mail.SendFailedException: Invalid Addresses;
nested exception is:
javax.mail.SendFailedException: 550 5.7.1 Unable to relay for milo@netgis.nl

at javax.mail.Transport.send0(Transport.java:219)
at javax.mail.Transport.send(Transport.java:81)
at alt.javax.mail.TransportImpl.send(TransportImpl.java:18)
at com.atlassian.mail.server.impl.SMTPMailServerImpl.send(SMTPMailServer
Impl.java:204)
----------------------------------------------------

After this error JIRA starts behaving strangely. It gets slower and slower and finally even seems to lose it's database connections. When I restart the tomcat instance, it runs good again.

We have been having this problem for 3 weeks now. Currently we are restarting JIRA daily, and this cannot be a good thing....

Please look into my error.

Is there a list with bugs concerning JIRA to hang? I wan't to check whether we are facing one of them.