Subscribe by Email plugin does not send e-mails

If I click on the "Send test e-mail" button no e-mail is sent to me. No logs in the subscribe-by-email-logs directory (which I had to create manually, is this correct?). No entry in the "sent e-mails" page.
Other plugins don't have problems sending e-mails.

Another problem: Upload of notification image doesn't work. Only a broken image is shown.

Please help!

Thanks, Martin