Pardon the intrusion into this conversation but thought it might be necessary to explain and advise.
Qualifications for this response: used to manage the core services for a wide area private network for 38 school districts in South Texas. Among those core services ... mail servers and mail gateways, DNS, web servers, and Moodle servers.
Can't use setup/information for moodle servers not on the same domain ... nor using same provider for Moodle hosting.
Clay has a moodle server on a Moodle Partner - MP's would have SMTP servers available for their customers.
Erdal you do not use an MP (and not suggesting you have to to use Moodle) but you do need to co-ordinate with your domain network and servers folks to get Moodle configured.
Clay's pointers to where in Moodle to find configuration screens is accurate, but
Erdal, you have to find out what's 'legal' for your domain ... i.e., the mail servers through which your Moodle server can relay mail.
For everyone's understanding ... Moodle is NOT a mail server but it does have modules that have Email features. Moodle programming might construct messages to be sent, but it doesn't actually send the mail. An MTA (mail transfer agent) or an SMTP server takes delivery of the message built by Moodle and it sends the mail.
First, in site admin -> Server -> php info
In the section of that screen for 'core" what does it say for sendmail_path?
Default is normally /usr/sbin/sendmail -t -i
SMTP is set to localhost
smtp_port is set to 25
Without changing Moodle configuration, Moodle attempts to use a php mailer script.
Message headers from Moodle forums, as an example, show this:
X-Mailer: PHPMailer Moodle 2014051202 (https://github.com/PHPMailer/PHPMailer/)
Which, by PHP's default, will attempt to use sendmail as the MTA (mail transfer agent).
Now let's got down to specifics for your situation @Erdal
Your Moodle site (www.moodle.) is under a domain: hitit.edu.tr
The mx (mail exchange) records in DNS for that domain are:
hitit.edu.tr. 3600 IN MX 5 mailgw.hitit.edu.tr.
hitit.edu.tr. 3600 IN MX 100 mail.hitit.edu.tr.
Your Moodle server is on the same network as the two servers above thus
what you need to do is communicate with the mail servers administrators on your
network and find out how to set up your Moodle server to relay Email.
It will involve an SMTP server setup - with hostname, protocol, and maybe a stmp user name with password.
What values and settings those would be NO ONE here on these forums can tell you specifically for the information collected in this response is of the outside network ... not the internal hitit.edu.tr network.
Again ... consult with network/mail server admins of the hitit.edu.tr network.
'spirit of sharing', Ken