This one took me a while to figure out and I was micrometers from posting a question here to try to find an answer but all the answers given to similar questions lead me to believe the problem was NOT inside of ISPConfig or postifx and it's settings. The situation was that I could send and receive email from my mail client using an ISPConfig hosted mail address but any attempt to send mail to that ISPConfig hosted mail address, from another mail server failed, with the error listed in the title. Checking all related threads here on the topic, the answer generally seemed to stem from checking /var/log/maillog and finding some evidence there but in my case, there were never any errors shown during attempts to send mail into the ISPConfig hosted mail server from an outside mail server. Of course the error messages I received from the external mail servers, of which I tried many, were pretty much worthless, other than the error listed in the subject, for which a googling turns up hundreds of possible causes. But, there not being any errors in the maillog file suggested that the external mail servers were never even reaching the ISPConfig hosted mail server so it seemed likely a network addressing problem. What turned out to be the problem, as likely most of you reading this already know, is that the DNS MX was not set up correctly. My knowing less than nothing to begin with and the domain registrar from whom I registered the relative domain names having little more info as to what to do other than, "Set the MX record to that needed by your mail server." didn't help at all! But, to cut what is ending up to be a long story short, the solution was setting the domain registrar's MX record to point to the ip address of the ISPConfig installation. After that, sending and receiving all the way around is working perfectly! I post this not to ask any questions but instead, to add the error message to the forum with a solution so that anyone in the future knowing as little as I do might find a solution using the forum search feature without having to ask. Even while I was having problems, I could see that ISPConfig is one great system but now that everything is actually working 100%, I've had time to dig into it and find even more things I absolutely love about it!!