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Introduction to E-mail Technologies

The process of message delivery by a mail server consists of the two phases. First, it detects address of the mail server that receives messages for the recipient (further on referred to as Recipient's Server, RS). Then, it connects to that server using the SMTP protocol and transmits the message to it.

Mail domain ( for the address; "alex" here is a mailbox in the domain) name normally is different from name of the mail server that receives messages for that address. At the moment when this text was being written, messages for were received by the servers and While computers with addresses and received no messages for no addresses. So, mail domain cannot be directly related to the mail server address, rather often messages are received by a computer with absolutely different name.

To find out RS address, a query is sent to the DNS service, which stores (besides other things) information about mail server that receives messages for each domain.

DNS is a distributed database. For example, DNS server stores all the information about, but it knows nothing about other domains, e.g. about The server stores information about domain, but it knows nothing about other domains. There is a server responsible for all .com domains, it keeps information about the servers that store domain information in the .com zone.

Your ISP's DNS has no information about or Therefore, when it receives a query about the name, it inquires the server responsible for the .com zone about address of the server that contains domain information for (it is, connects to that server and returns the answer to you. This way of query execution is referred to as recursive.

We are not going to enlarge upon DNS technology here (it is well described in numerous public sources). The fact important for us is that a query to the DNS service might come through several DNS servers scattered over the globe before you get the answer. And, after all, it's domain owner who is responsible for storage of information about it.

There is a common practice of caching DNS queries. Normally, DNS server remembers the recursive query results for a couple of days in order to reduce the DNS server loading to ensure faster query execution (the information about maximum possible number of days for the result caching is contained into answer to a query). This means that when DNS record suddenly changes, it might take several days before caches of other DNS servers on the Internet are updated and their users get the up-to-date information.

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