strange e-mail behavior

[Bart]

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.

[Ruud]

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

[Bart]

the Mailer Daemon message starts like this:

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.