error: GPG validation failed!
The update downloaded successfully, but it was not signed with a trusted GPG
key.  Instead, it was signed with the following keys:

    856AA88A

Perhaps you need to import the channel's GPG key?  For example:

    wget http://spamassassin.apache.org/updates/GPG.KEY
    gpg --import GPG.KEY

channel: GPG validation failed, channel failed