(1) Your DNS on the name server is not configured correctly or:
(2) The sendmail.cf files is misconfigured.
Earlier you stated that DNS appears to be working; however, your troubleshooting techniques could be more methodical (step-by-step). You need to describe the configurations, the servers, the clients in detail.... to do otherwise is an endless circle of frustration for we. We can't really efficiently help because you will not take the time do describe the exact configuration, hostnames, client, server processes, etc.
that's the equipment. There's one single mail server for smtp. It's got a permanent Internet connection so it doesn't need to relay or forward mails to an outstanding mailsever. It should directly send the mails to the addressed domains.
My ISP set my two DNS Primary and secondary as usual.
that's the whole stuff.
I think your right about the dns problems. It isn't directly on dns resolving. but sendmail needs for sending to diffrent domains through itself as fqdn. The errors refers to mailhost.mydomain.com, what isn't already defined in my dns, so sendmail couldn't find it, actually itself.
I dont know why but there are two dffrent cases about sendmail, I guess.
a) send mail to local domain. It don't look for mailhost.whatever.com, just localhost. No dns query!
b) ougoing mail to a differnt domain. Look for dns entrie on its own domain in dns - mailhost.domain.com what it couldn't find, so the fatal error happend and breaks at mailhost.mydomain.com host unknown!
Is my idea possible?
I don't know why sendmail needs to look for itself dns entrie to resolv itself.
Your details are not sufficient. To solve the problem we need the complete details. For example: What is the FQDN of the machine where you are to trying to configure sendmail ?
I set up the dns server, in a few hours should be all up! But why do I need this just for using smtp? For pop3 is clear! To resovl the server itself can be harcoded in host, isn't it?
It means what it says. Your system tried to contact mailhost.sharkms.ch. It was up but was not listening to port 25. They may have been reconfiguring their mailserver, or maybe the system was overloaded or maybe its mail queue area was full. Anyway, you should be running a sendmail daemon that will retry the mail. These usually go through after a while. Check the mail queue in a few hours.
yes I though the same, but it's a differnt problem.
I changed the sendmail.cf to an old that I configured.
It's wokrnig now!!!!! I'll look for the changes.
I will publish it and all my configuration for all those other poeble who have trouble with sendmail to make it a bit easier for them!!!