I m not able to send emails to some addresses. I get error message: "Helo command rejected:need fully-qualified hostname"

As the error message states, you NEED to have a fully qualified domain name in your HELO/EHLO greeting. The reason you get this error sometimes is because not everyone is strict about this. This is one tactic I personally use to filter out spam as spammers rarely have RFC compliant mail servers.

I think you can fix this a couple ways.  The first to try is setting the hostname variable on the server. If you login as root and type: "/bin/hostname" it should spit out your server's name (the unqualified one). You should set that to the full dns name of the server:

# /bin/hostname mailserver.yourdomain.com

Then restart sendmail. It should start announcing itself

The other setting (if I remember correctly) is in the /etc/mail/sendmail.mc file:

MASQUERADE_AS(`mailserver.yourdomain.com')dnl

Tweak that line, backup your /etc/mail/sendmail.cf file, recreate the sendmail.cf file:

# m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

and restart sendmail.

Hi,

The mail relay is windows server 2003, how can I do such operations on this OS?

But just to make sure, you mean the name of the server should be the same as the domain name?

oh...I assumed since you were in the sendmail section, you were using sendmail.

Are you using exchange or just ms smpt (iis) ?

Actually i have an exchange server, which relays messages to the mail relay, which is smtp iis. All incoming messages are also realy to  the mail server, and the exchange server retrive incoming mails from there. Actually, the relay is in the DMZ of the firewall and the exchange server is in the internal network.

the solution is on
https://www.experts-exchange.com/questions/22515477/Fully-Qualified-Domain-Name.html