Please see/report problems in the TYPO3 Bugtracker under project tx_directmail.
You may get support in the use of this extension by subscribing to news://news.netfielders.de/typo3.projects.direct-mail.
SMTP / PEAR
There is a bug in PEAR class in sending email to user with special characters in the name field. Example: it won't send email to a user, whose name is test-directmail.
More of this bug please see PEAR Bugtracker