Problem

After configuring the mail server in Confluence to use SMTP  with port 465, sending a test email fails with the following log:

The following appears in the Atlassian-Confluence.log

2016-02-02 02:57:04,496 ERROR [http-nio-8090-exec-1] [atlassian.confluence.jmx.JmxUtil] unregisterBean Unable to unregister object with name [ Confluence:name=MailServer-SMTP Сервер ]  due to : javax.management.InstanceNotFoundException: Confluence:name=MailServer-SMTP Сервер
 -- referer: http://109.206.188.107/confluence/admin/mail/editmailserver.action?id=1454367622001 | url: /confluence/admin/mail/doeditmailserver.action | userName: cnf | action: doeditmailserver

Affects Confluence 7.16.x and earlier. 

Cause

Port 465 shows up in Appendix A of the 1996 non-standard The SSL Protocol Version 3.0 as "Simple Mail Transfer Protocol with SSL". Unfortunately, it's not registered for SMTPs. It's registered for URD - "URL Rendezvous Directory for SSL" by Cisco. The recommended approach, at least for authentication, is to use STARTTLS encryption on port 587.

Port 25 can be used but it is for server to relay messages to one another. Port 587 is recommended because this way ISPs can block outgoing SMTP on their networks but still allow users to send email to another mail server.

Resolution

See Configure the Mail Server to Use GMail.