Moodle - Forums not sending mail notifications - cron

Question

I don't succeed in making Forums Moodle module sending mails for posts notification.
Although mails sending is working right in Moodle platform, when it comes to Forums module it doesn't work. I executed cron.php from the browser and I got:
Starting activity modules
Processing module function assignment_cron ...done.
Processing module function chat_cron ...done.
Processing module function forum_cron ...Processing user 3
Sending ERROR: SMTP Error: Data not accepted.
Error: mod/forum/cron.php: Could not send out mail for id 13 to user 3 (test.test#test.org) .. not trying again.
post 13: Re: test4
Sending ERROR: SMTP Error: Data not accepted.
Error: mod/forum/cron.php: Could not send out mail for id 14 to user 3 (test.test#test.org) .. not trying again.
post 14: Re: test4
Processing user 4
Sending ERROR: SMTP Error: Data not accepted.
Error: mod/forum/cron.php: Could not send out mail for id 13 to user 4 (test.test#test.org) .. not trying again.
post 13: Re: test4
Sending ERROR: SMTP Error: Data not accepted.
Error: mod/forum/cron.php: Could not send out mail for id 14 to user 4 (test.test#test.org) .. not trying again.
post 14: Re: test4
....
post 19: Re: teacher topic
0 users were sent post 13, 'Re: test4'
0 users were sent post 14, 'Re: test4'
0 users were sent post 15, 'new topic test'
0 users were sent post 16, 'Re: new topic test'
0 users were sent post 17, 'Re: new topic test'
0 users were sent post 18, 'teacher topic'
0 users were sent post 19, 'Re: teacher topic'
Starting digest processing...
Cleaned old digest records
done.

Answer 1

In moodle 1.9 or 2.0 go to Site administration ► Server ► Email to check the SMTP settings.
Click on the moodle docs link at the bottom of this page and it will take you to moodle documentation, where it talks about Email settings, SMTP settings in particular.
If you cant find the link to moodle docs, http://docs.moodle.org/21/en/admin/setting/mail