it may well be linked to the content of the email
other forums on this subject are a bit vague but it suggests that the recipient has some sort of mail protection regarding possible virus content
of the email you are trying to send. i had a similar problem useing incredimail whereby the virus was in the letter style itself and any scan
useing all the tools you have as well did not show anything as you managed to send through hot mail i am not sure on that.