The bug lies on line 380 of class "MailLogger", where "mailer.setEnableStartTLS(values.ssl());" should be replaced by "mailer.setEnableStartTLS(values.starttls());", so that the "MailLogger.starttls.enable" system command-line property is properly taken into account. With the current state of the source code, it is not possible to use the Gmail SMTP server for logging the result of an Ant script execution!
Sorry, I knew I shouldn't have committed a change I couldn't test myself. Fixed with svn revision 1065204 Thanks!