helloThat is not enough information to help. It is always better to give too much information than not enough. A good start would be to tell us which OS, and mail program versions you are using, and exactly what errors you are seeing. Perhaps even some snippets from the log files to see what the failure is.
helloThat helps a little more, but what exactly are you seeing and please include parts of the log files (/var/log/exim/mainlog and /var/log/exim/rejectlog) that pertain to this failure. Without those, I am sure we could list 1000+ possible things that could cause sending emails to fail.