HDR From: The Post Office HDR Sender: mailer-daemon SUB Subject: Usage of IP-number as an address (error) The following message arrived with illegal envelope data, typically a mangled address that doesn't obey the RFC822/976 protocol specification. Some BSD sendmail systems allow users to use IP-addresses as address for delivering mail. Such kludge format should NEVER be used. Furthermore proper format for it is not: foo@12.34.56.78 but: foo@[12.34.56.78] Again: THIS KLUDGE SHOULD NEVER BE USED! Your message is being returned unprocessed. The following annotated envelope headers illustrate the error(s):