When you try to send an email, you get the following error message:

An error has occurred with sending the test email:
com.atlassian.mail.MailException: javax.mail.NoSuchProviderException: smtp
at com.atlassian.mail.server.impl.SMTPMailServerImpl.send(SMTPMailServerImpl.java:152)
at com.atlassian.jira.web.action.admin.mail.SendTestMail.doExecute(SendTestMail.java:72)
at webwork.action.ActionSupport.execute(ActionSupport.java:153)
at com.atlassian.jira.action.JiraActionSupport.execute(JiraActionSupport.java:46)
at webwork.dispatcher.GenericDispatcher.executeAction(GenericDispatcher.java:131)
at com.atlassian.jira.web.dispatcher.JiraServletDispatcher.service(JiraServletDispatcher.java:186)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
...

This can be solved by removing activation.jar and mail.jar from common/lib directory. After removing the two files, you will need to restart JIRA.