Jed: Your ISP is correct. However, you are partially correct. An SMTP session consists of a server listening on a port (1.2.3.4:25) and then a client connecting to that socket and sending a properly formatted message to it. This can be done manually if you learn the right SMTP commands (HELO, MAIL, RCPT, DATA) and their arguments. Internet e-mail works in conjunction with the DNS (___DOMAIN___ name system) to route messages. For instance, clue.denver.co.us is a valid domain. We start at the .us top level domain, then the .co subdomain, then the denver subdomain, then the clue subdomain of that. There is a hierarchy of domains which is what makes the DNS work. A mailer that wants to send a message to so-and-so@clue.denver.co.us can walk the DNS tree until it gets the MX record for "clue.denver.co.us" from the DNS server for that domain. So consider how the DNS might work for an IP address. Let's say that you wanted to send an e-mail to person@2.3.4.5 . Would "5" be the top- level domain? How is your mailer supposed to walk the DNS tree to find the MX for a subdomain in the "5" domain? (AFAIK single numbers are not valid top-level domains; you can use .COM .NET .ORG .US etc. but not numbers.) Remember that in the DNS, IP addresses are actually represented in the inverse notation - ie.e, "5.4.3.2.in-addr.arpa" would be the domain. It is possible to put MX records in the inverse zone files for the DNS for inverse domains, i.e., the "5" record in that inverse zonefile could have an MX record pointing to itself. Then you could send an e-mail to person@5.4.3.2.in-addr.arpa if you wanted to. (Assuming that the mail program on that system was configured to recognize that domain name as well.) In practice it is very unusual to find MX records in the inverse zones since everybody uses the domain name. The integration of sendmail (and internet mail in general) with the DNS is by design. It was not just an arbitrary decision on the part of some software developers - that's actually how it is supposed to work. If you don't want to use the DNS you can just telnet to port 25 on the server and send some SMTP commands. Regards, JimO > Greetings folks. > Maybe I'm confused, or just remembering simpler times. > Due to some dns resolution problems, I tried sending an e-mail directly to > a mail server I identified using a dns mtx query, i.e. theperson@n.n.n.n - > which bounced, because of a host lookup failure at my ISP. Now, I can > understand how it might bounce at the other end, because it might rely on > having the domain name component for identifying the recipient, if the > mailserver is handling multiple domains. But I just don't buy the MTA > being unable to recognize a dotted-quad IP address and using it. > I really don't want to become an SMTP guru, or a sendmail or postfix guru > either. So maybe somebody has nice URL where this is explained, or could > tell whether my ISP is blowing smoke, or whatever. Here's what they said: > < quoth > > All mail is handled by name, not by IP address. This is an internet-wide > standard that everyone adheres to. You cannot send email to an IP address > only. I have talked with our system administrators about this, and I > have done some testing myself, sending to different email addresses at > the mail server's IP address. I have sent through many different mail > servers and they all have this same problem. Mail absolutely has to be > handled through DNS, so sending directly to the IP, instead of the name, > is not possible. > < end quoth > > But really, isn't sending mail, well using SMTP anyway, just a matter of > having a server/daemon listing on the proper port, and sending a properly > formed message to it? > TIA. > jed > -- > "Those who expect to reap the blessings of freedom must, like men, > undergo the fatigue of supporting it." > - Thomas Paine "The price of liberty is eternal vigilance." -- Jim Ockers (ockers@ockers.net) Contact info: please see http://www.ockers.net/ Fight Spam! Join CAUCE (Coalition Against Unsolicited Commercial Email) at http://www.cauce.org/ .