Legitimate email marketing firms that do not remove out-of-date Hotmail addresses face being added to Microsoft's spam filter lists, according to an industry analyst.
Richi Jennings, an analyst at Ferris Research, claimed that, after a year of disuse, Microsoft may use old Hotmail accounts as spam traps.
"This means that email sent to old Hotmail addresses may be used as samples to help train spam filters for Hotmail, MSN, FrontBridge, the Outlook Junk filter and other services," he said.
"Microsoft is by no means unique in doing this, but is said to be more aggressive than most."
Jennings advised legitimate marketers to detect and eliminate email addresses that bounce back from their lists.
"If a receiving mail system consistently tells you that an address is bad, remove that entry from your lists," he said.
"If you don't, your IP range can be blacklisted and/or your message content will seem more 'spammy'."
Jennings warned that it used to be simply bad manners for a sender to continually send mail to nonexistent addresses, but that it is now potentially self-destructive.