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 Are Dead Addresses Really Dead? |
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Are Dead Addresses Really Dead?
What is a dead address? First, it is an address with wrong syntax. Such address is always
dead. Second, it is an address from an unbought domain or a domain that was not
paid in a due time. Such an address might become "live" again if
somebody buys a domain. Third, it is an address from a domain that has been
bought but hasn't been registered in DNS, or no mail server addresses are
registered for it in DNS. Some domains were bought only because of their names,
their only function is to send visitors to main site of a company; normally, no
e-mail address is needed for a domain of that kind. Such addresses may come
alive at any moment once domain owner's plans change. With some domains, DNS
addresses refer to other machines which are not DNS servers any more or are off
due to some technical or managerial problems. But tomorrow the error will be corrected,
DNS server caches will be updated and the addresses will come back to life in
some three days.
Both domain life and life of an e-mail address in that domain are rather relative. Some
addresses die irreparably, some will become alive again in a day, some will
come back to life in a month. Somebody will buy a dead domain in a year, and
the address webmaster@ will become valid again.
We have done the following test. We checked a list of addresses, and then rechecked the
list of invalid addresses. This new check revealed 0.65% of existing addresses.
Check of the remaining list of invalid addresses revealed 0.3% of existing
addresses again. We repeated the check again and again, and with every new
check the number of existing addresses found was several times
smaller — down to hundredths of a percent. In total,
repeated checks revealed that about 1.1% of the addresses in the list of
invalid addresses actually existed. We run the checks continuously for two
days.
The list of revived addresses was checked by HSV and only 30% of the addresses existed, 70%
were found invalid again. When remaining revived addresses were checked again a
day later, only 33% proved actually existing. So, finally, as little as 9% of
the revived address list remained. It made 0.1% of the initial list of invalid
address list.
It is possible to assume that about 1% of invalid addresses found are unstable,
"half-dead". And 90% of unstable addresses remain unstable for at
least several days.
Our investigation delivered a simple explanation of why most of unstable addresses
behave so. Software of many DNS is configured so that when replying to a
recursive query is impossible (see the section "Introduction to E-mail
Technologies") they return the result "Server not found". Actually, another
reply code would be more adequate – then, High Speed Verifier could repeat the
query through another server. Some DNS servers do so, and HSV makes five
attempts at checking an address through different DNS servers. Finally, one of
the attempts succeeds, or the address is marked as unchecked.
So, if a DNS server is connected to the web through an overloaded, unstable channel,
e-mail addresses on the domain it is responsible for are unstable, too.
As we've
already noted, dead address is a relative definition, the situation might
change over time. If you are going to use a mailing list with millions of
addresses, it would be advisable to exclude all the addresses marked by HSV as
unchecked or invalid. Because messages to 99% of such addresses will be never
delivered; and 1% of the messages will be delivered after hours of attempts.
But don't
discard the list of invalid address — if you have a million of dead
addresses, you have a chance to retrieve about ten thousand of revived
addresses from it in a week. |
