Identifying the php scripts that are sending mail is the first step to protecting yourself against would-be spammers. Follow these steps to find the domains on which these scripts are running using Postfix:
Following command can be used to list the kernels in centos 7
# egrep ^menuentry /etc/grub2.cfg | cut -f 2 -d \' Linux Server, with Linux 3.10.0-123.el7.x86_64 Linux Server, with Linux 3.10.0-123.4.4.el7.x86_64 Linux Server, with Unbreakable Enterprise Kernel 3.8.13-35.3.2.el7uek.x86_64 Linux Server, with Unbreakable Enterprise Kernel 3.8.13-35.3.1.el7uek.x86_64 Linux Server, with Linux 0-rescue-d3e0313c0f6d48a0bb72495d2x32r1
You can use grub2-set-default command to set default boot kernel, to set first kernel in grub2.cfg as default run and reboot.