strange e-mail behavior

I am not sure where to put this question.
I am running a website with a forum (using the Helpdesk extention) of roughly 1000 members.
After the recent update to 2.10, the e-mail system of Contao behaves very odd. I a new member subscribes, he receives an automated "activation" e-mail. This feature is basically copied from the "Music Academy" template that was included with Typolight when I first started building the website.
Since the upgrade to 2.10, I started receiving these strange Mailer Deamon warnings. Apparently the system sends these activation e-mails not only to the newly listed member, but sends copies of the same e-mail to many different e-mail addresses. At least a number of these addresses are invalid, hence a Mailer Deamon warning filled with warnings about undeliverable e-mails. The strangest thing is that I have no clue where those invalid e-mail addresses come from. They do not belong to any of the existing members.
The same thing also happens when I sent an e-mail message to myself (for testing purposes) through the member profile form. Also this e-mail was copied and sent to a number of invalid e-mail addresses. I don't know if the message was also send to valid recipients.

Does anyone have a clue what could be causing this problem?

Thanks,
Bart.

Is there information inside the log. And what do the email and error logs say? (email.log and error.log)

the Mailer Daemon message starts like this:

Quote:

---

Hi. This is the qmail-send program at pro001.watsnel.nl.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

<mmelibosky@parkplaza.cl>:
200.71.193.242 failed after I sent the message.
Remote host said: 550 Denied by policy

<mjimenez@uai.cl>:
192.245.60.24 does not like recipient.
Remote host said: 550 5.7.1 Su Ip se encuentra bloqueado por nuestro sistema Antispam Barracuda. Su mensaje no puede ser entregado al destinatario.
Giving up on 192.245.60.24.

<notengo@laq.com>:
74.125.79.27 does not like recipient.
Remote host said: 550-5.1.1 The email account that you tried to reach does not exist. Please try
550-5.1.1 double-checking the recipient's email address for typos or
550-5.1.1 unnecessary spaces. Learn more at
550 5.1.1 http://mail.google.com/support/bin/answ ... nswer=6596 r55si1654836eeh.117
Giving up on 74.125.79.27.

---

And so it goes on for a while.

The error log contains no entries for the dates when the e-mail issue occured.
The e-mail log contains lines that state:
"an e-mail has been sent to [e-mail-address]", where [e-mail-address] are the valid e-mail addresses of the people that indeed needed to receive an e-mail. There are no references to the unexpected e-mail addresses from the Mailer-Daemon message.

Thank you for looking,
Bart.