The jenkins-ci.org mail server does send a fully qualified hostname on SMTP HELO, which causes mails to get rejected by some mail servers. I personally did not get the confirmation mail for cweiske@cweiske.de because the server rejected the mails.

Solution: Instead of cucumber, send a hostname like cucumber.jenkins-ci.org

SMTP log:

NOQUEUE: reject: RCPT from 63-246-20-93.contegix.com[63.246.20.93]:
 504 5.5.2 <cucumber>: Helo command rejected: need fully-qualified hostname;
 from=<admin@jenkins-ci.org> to=<cweiske@cweiske.de>
 proto=ESMTP helo=<cucumber>