From Zimbra :: Wiki
(a) Run this command on the old server
su - zimbra
/opt/zimbra/libexec/sa-learn -p /opt/zimbra/conf/salocal.cf.in --dbpath /opt/zimbra/data/amavisd/.spamassassin/ --siteconfigpath /opt/zimbra/conf/spamassassin --backup > /tmp/zimbra_q.backup
(b) copy the file /tmp/zimbra_q.backup to the new server
(c) Run this command on the new server
su - zimbra
/opt/zimbra/libexec/sa-learn -p /opt/zimbra/conf/salocal.cf.in --dbpath /opt/zimbra/data/amavisd/.spamassassin/ --siteconfigpath /opt/zimbra/conf/spamassassin --restore /tmp/zimbra_q.backup