I found the following article on Parallels website for this issue
http://kb.parallels.com/en/116927
Resolution
Disable the mail service for the domain.tld subscription:
Go to Subscriptions > domain.tld > Mail > Change Settings.
Uncheck Activate mail service on domain and click OK.
You can also disable the mail service on the subscription using the command-line utility mail:
/usr/local/psa/bin/mail --off domain.tld
To disable the mail service for every subscription on the server, use the following command:
mysql -uadmin -p`cat /etc/psa/.psa.shadow` psa -Nse"select name from domains where parentDomainId=0"|while read i; do /usr/local/psa/bin/mail --off $i && echo "Mail service for $i subscription has been disabled" ;done
I wish I knew how to do this for every subscription through phpMyAdmin my running a mySQL query...