The use by spammers of dictionary attacks means those whose email address begins with a less common first character are liable to get less spam.
Research by Richard Clayton of Cambridge University was initially reported as establishing that usernames beginning with A are likely to get more spam than those sporting a Z.
Those with email addresses starting with A - "aardvarks" - got 35 per cent spam, but "zebras" received a mere 20 per cent.
That's true, but it's only part of a bigger picture, Clayton clarified on Friday.
"The point being that the effect I am describing has little to do with Z being at the end of the alphabet, and A at the front, but seems to be connected to the relative rarity of zebras," Clayton explained.
M and P are popular first letters for people’s names. Each get around 42 per cent spam, more than those whose usernames begin with A.
Clayton based his analysis on email traffic logs maintained by Demon Internet. The ISP filters automatically filters out some spam based on its origin, so the percentage of spam to real email (ham) received by Demon users is better than commonly quoted figures that four in five email messages or more circulating on the net are junk mail.
The study revealed a disparity between the proportions of spam received by active addresses with different first characters.
The actual volume of spam a user receives will depend on factors such as whether they have been careful not to post their email address online or to avoid responding to dodgy offers. Clayton's research shows that even averaging out different behaviours, something as simple as the first character in an email address can have a big effect on the junk mail volumes a user receives.
"The most likely reason for these results is the prevalence of 'dictionary' or 'Rumpelstiltskin' attacks (where spammers guess addresses). If there are not many other zebras, then guessing zebra names is less likely," Clayton explained.
"Aardvarks should consider changing species — or asking their favourite email filter designer to think about how this unexpected empirical result can be leveraged into blocking more of their unwanted email," he added.
Clayton presented his research in a paper (pdf) at the fifth conference on email and anti-spam in Mountain View, California, earlier this month. ®