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 Introduction to E-mail Technologies |
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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
(mail.com for the address alex@mail.com; "alex" here is a mailbox in the mail.com
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 alex@mail.com were received by the servers mail-com.mr.outblaze.com and mail-com-bk.mr.outblaze.com. While
computers with addresses mail.com
and www.mail.com
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 ns1.outblaze.com stores all the information about mail.com, but it knows nothing about
other domains, e.g. about hotmail.com.
The server ns1.hotmal.com stores
information about domain hotmail.com,
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 mail.com
or hotmail.com. Therefore, when it
receives a query about the name mail.com,
it inquires the server responsible for the .com
zone about address of the server that contains domain
information for mail.com
(it is ns1.outblaze.com), 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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