Below is a copy/paste of our KB page regarding somehow unexpected behaviour of JavaMail. We should investigate and if possible disable the fall-back to localhost.

Symptoms

1. Mail Servers page displays correctly user defined SMTP server (different from localhost)
2. The following appears in the atlassian-jira.log:

   com.atlassian.mail.MailException: javax.mail.MessagingException: Could not connect to SMTP host: localhost, port: 25;
     nested exception is:
       java.net.ConnectException: Connection refused
       at com.atlassian.mail.server.impl.SMTPMailServerImpl.send(SMTPMailServerImpl.java:169)
       at com.atlassian.jira.web.action.admin.mail.SendTestMail.doExecute(SendTestMail.java:93)
   ...
   Caused by: javax.mail.MessagingException: Could not connect to SMTP host: localhost, port: 25;
     nested exception is:
       java.net.ConnectException: Connection refused
       at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:1391)
   ...
   

Cause

JavaMail library attempts to connect the server specified by user and on failure it defaults to localhost and port 25.

Resolution

Ensure that valid Host Name and protocol parameters are specified in SMTP Mail Server section of Mail Servers page