The problem is with sending mails from the local machine to my external ISP mail account.
I got the xxx.dyndns.org domain. The local machine name is aaa. It seems that when the sandmail sends the mail it puts in the FROM field th username@aaa.xxx.dyndns.org address. Then the remote smtp server answers that aaa.xxx.dyndns.org does not exist.
I do not know how to force sendmail to use only the domain name In the FROM field, without the machine name.
Yes. It is configued in /etc/hosts file.
You can resolve aaa.xxx.dyndns.org within the internal network.
However only xxx.dyndns.org is resolvable on the internet.
.I've checked this site already. but with no solution
I've tried tcpdump and found out that my sendmail send the wrong domain in both EHLO and MAIL smtp commans.