I've built a system for a client to send html based emails. I'm having a problem that any external email address is rejected by the Send Email function. Looking at the logs I can see the following: I've spoken to the client's IT support who manage their email server and checked all the details used. It appears this is caused by javamail, but we're not sure how to get around this issue. Has anyone had a similar issue, and how did they get around it?