Failing to send an email externally

[alzamabar]

Hi all, I'm using James as mail server and it works fine both as POP3 and SMTP server, meaning that I'm using it with the most commons mail clients.
I'm trying to send an email directly from Ant with the mail target but I receive the following error:
-- BEGIN --
mail:
[mail] Failed to initialise MIME mail
[mail] Sending email: [ANN] A new version of Jamailet is available for downlaod
[mail] Failed to send email
BUILD SUCCESSFUL
Total time: 3 seconds
-- END --
The strange thing is that if I specify myself as to.recipient the email is delivered without no problems.
Any idea?