Since I am one of those that really like that poster in a forum confirms that the solutions actually works, with any modifications that was needed I will post what I did now, correct me please if this wasnt the right way, anyhow here goes:

I did su - zimbra to do all commands as the zimbra user, I created the gnupg dir (since I got a permission denied when gpg tried to do it itself); as root user, mkdir /opt/zimbra/.gnupg && chown zimbra:zimbra /opt/zimbra/.gnupg, then su - zimbra to become zimbra user again.
gpg --keyserver pgp.mit.edu --recv-keys BDE9DC10
cd /opt/zimbra/.gnupg
Now, export our key alone from zimbras's public key ring by running the command
gpg --armor -o pub.gpg --export BDE9DC10

Import the public key into sa-update's trusted public keys by running
sa-update --gpghomedir=/opt/zimbra/.gnupg --import pub.gpg

Got an error when I wanted to use the --channel updates.spamassassin.org also, so had to download the gpg key from spamassassin.apache.org
wget http://spamassassin.apache.org/updates/GPG.KEY

and import the key with sa-update command
sa-update --gpghomedir=/opt/zimbra/.gnupg --import GPG.KEY

then last, import the updates from the 2 channels ( --channel saupdates.openprotect.com --channel updates.spamassassin.org)
sa-update --gpghomedir=/opt/zimbra/.gnupg --updatedir=/opt/zimbra/conf/spamassassin --gpgkey D1C035168C1EBC08464946DA258CDB3ABDE9DC10 --channel saupdates.openprotect.com --channel updates.spamassassin.org