Share by email functionality doesn't work

When I click 'Share by email' I get a dialog that I fill out and then click 'send'. The dialog returns with an error stating that "The message could not be sent to xxxx: email server error.". There is, however no error nor exception to be found in the catalina.out of the wiki nor in the logs of the mailserver and I don't see any port 25 traffic while tcpdump'ing the outgoing interface on the (wiki)server. The server is reachable and accepts mail from the wiki server as I can send mails from commandline (mail) and using the "Test Send Message" extension in the Sandbox.