When we ask to send a mail with the settings, it shows the following error: "Could not send settings email".
The message is accurate: the mail isn't sent.
Tests with mail() function directly from PHP cli work. So, yes, PHP from www-data can send mails to external domains.
On previous software version + Debian Wheezy, all worked like a charm.
This only affects vma.nasqueron.org: all other tests to send mails work correctly.