I have a user who receives e-mail sent to an Exchange 2007 e-mail distribution list, but she's not a member of that list. I have verified via the Exchange Management Shell that she is a member of one distribution list, which happens to be a mail enabled security group. The commands I used to verify this are:

Get-DistributionGroup | where { (Get-DistributionGroupMember $_ | foreach {$_.name}) -contains "yourUserName" }

Get-DistributionGroup | where { (Get-DistributionGroupMember $_ | foreach {$_.PrimarySmtpAddress}) -contains "yourUserPrimaryEmailAddress" }

Any ideas as to why this could be happening?