Same here.

You can try:
tail -200 /var/log/syslog | grep mail

(or some similar pattern that works out for you) to locate the problematic lines. In my case:

Apr 7 23:20:51 cachogordo postfix/qmgr[2287]: 2D750378714: from=<xxx@gmail.com>, size=1123, nrcpt=1 (queue active)
Apr 7 23:20:51 cachogordo postfix/error[8296]: 2969B379145: to=<xxx@gmail.com>, relay=none, delay=0.13, delays=0.06/0.01/0/0.06, dsn=5.0.0, status=bounced (gmail.com)
Apr 7 23:20:51 cachogordo postfix/error[8298]: 2D750378714: to=<yyy@gmail.com>, relay=none, delay=0.12, delays=0.09/0.01/0/0.02, dsn=5.0.0, status=bounced (gmail.com)

(xxx and yyy conceal the real addresses of course)

So here it seems to be bounced by gmail.com
I set up gmail's server data in the Site Preferences / Mail Setup values:
mail.google.com
"username"
"password"

But still get the same output.
Any idea?

Hello,
my installation of XOOPS works fine except of the email sending (confirmation emails on registering).

Where can I find info of how XOOPS manages email sending?
Is it relying on postfix, sendmail or something similar?

Thanks a lot