[exim] Smtp not possible from some clients

Hello,

I have a problem with the smtp connection from some clients web application which is using a smtp connection to send mails. The output only says that no smtp conenction is possible, therfore I do not have a detailed error message.

On the server side I searched the firewall entries, the receiving of the TCP packages and the logs. I only found the following log entrie, when the Client is connecing to the smtp server which seems to be failed.

Message:
no host name found for IP address xx.xx.xx.xx

I searched very long time but do not found a real solution I can do on my side. I tried to whitelist the ip address or Change the host_lookup option but the issue still occur. Also I checked installed fail2ban logs but there is also no helpful entry. The Connection from outher Clients is working so it seems to be not a general issue.

Can someone help to solve the problem?

Thanks