RESOLUTION
First of all, you have to back up mail contents on Plesk server you are going to migrate mail from./usr/bin/mysql -r --batch -N -uadmin -p`cat /etc/psa/.psa.shadow ` psa -e"select id from domains;" > domains_id
/usr/local/psa/bin/psadump --nostop --nostop-domain --tar-ignore-failed-read --domains-ids=domains_id --backup-only-mail --force -f - | split -b 2000m - linux2mail_dump. Transfer the backup file to Mail server.
scp linux2mail_dump.* root@IP_ADDRESS: Restore the backup file on Cmail server.
cat linux2mail_dump.* | /usr/local/psa/bin/psarestore -f - -m ip_map -s shall_map Once restoration is completed, you can assign Plesk server to Cmail with "Migrate mail content" button unchecked.