Because there was a bug adding user hestiamail. You need to remove the mail address added to the hestiamail user in /etc/passwd file.
As root, execute below commands to fix the issue:
sed -i -E 's/(^hestiamail.*:)([^:]*@[^:]*)(:.*)/\1\3/' /etc/passwd
/usr/local/hestia/bin/v-update-user-counters