centos 7 – Why does a mail header report a different hostname from the configured in the mail server


When I send an mail to Hotmail using the SMTP server mail.example.com and then when I received it and see the original message, I can see this in the header:

helo=vps.example.com;
Received: from vps.example.com (204.93.xxx.xxx) by DB5EUR01FT037.mail.protection.outlook.com (10.152.4.212) with Microsoft SMTP

In mail server I have configured mail.example.com as the hostname and even, when the e-mail is sent, it is sent through the mail.example.com. Both names has the same IP address, however, I want the helo and the received from to be mail.desytec.com

The SMTP server is Exim and as I told, if I issue a hostname command, I see it is mail.example.com.

Besides that, if I telnet localhost 25 in the same server, the SMTP greetings shows correctly mail.example.com

How can I change it?

Thanks

Jaime