After upgrading our Jenkins server from 1.636 to 1.638 we don't get any unstable mails anymore. The check-box within the job on "Send e-mail for every unstable build" is set (also checked the config.xml).

If I take a look into the console output of the job I'll see:

Sending e-mails to: myAddress@myDomain.com
Finished: UNSTABLE

But nothing reaches our mail server. But it cannot be a real configuration issue, cause failing jobs produce the following output:

Sending e-mails to: myAddress@myDomain.com
Finished: FAILURE

And these mails will be send and reaches my inbox.