Are Dead Addresses Really Dead?
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.