WordPress MU

The mails sent for signup's activation have no "from" address, or not a valid adress.

I found that in the logs of my sendmail, and all mail are rejected by destination server :(

I checked, the site admin option "Site Admin Email" which is said to be used for this kind of mail : "Registration and support mails will come from this address. Make it generic like "support@example.com" .

And does it exist?

yes, the address I put as "site admin email" exists.
It has been validated normally when I installed WPMU.

The address I put to try the signup also exists.
In my /var/log/mail.info :
The destination server block the mail and give as a reason : "Sender address must resolve"

So yuo can normally send and recieve emails at thsoe accounts, correct?

if so, do a reverse DNS on your server / IP address. Google is your friend for that one. :)

just one more precision : the "from" is "www-data" according to /var/log/mail.info

and I installed BuddyPress, but I don't think it's linked to that plugin.

I'd turn it off and try again, just in case.

Also, search the entire database for "www-data". That "www-data" is the default Apache username.

I tried what you said :
disabling BuddyPress did not solve the problem.

I search in the whole database thanks to PhpMyAdmin : no www-data.
and actually, you give the answer : this is the defaut Apache username.
So there was 99% chance that the message is sent by wordpress, and Apache put its defaut "from" because WP did not set any value for this field.

In the table "wp-users", I have only one user :

+----+------------+------------------------------------+---------------+------------------------+----------+---------------------+---------------------+-------------+--------------+------+---------+
| ID | user_login | user_pass                          | user_nicename | user_email             | user_url | user_registered     | user_activation_key | user_status | display_name | spam | deleted |
+----+------------+------------------------------------+---------------+------------------------+----------+---------------------+---------------------+-------------+--------------+------+---------+
|  1 | admin      |  # # # encripted-password # # #    | admin         | admin#@#blog-recette.com | http://  | 2008-11-22 12:57:19 |                     |           0 | admin        |    0 |       0 |
+----+------------+------------------------------------+---------------+------------------------+----------+---------------------+---------------------+-------------+--------------+------+--------

(I just add the '#' in the adress, but the adress is good)

I have 2 users in the wp-signups .... which are the ones I desesperatly trying to create.