Snapt images (depending on configuration) may have the sendmail daemon running on port 25 on localhost. This can cause issues when your Balancer group or Frontend is bound to 0.0.0.0:25 and there are two options
Option 1: Specify the IP to listen on
You may want to set an IP for your Balancer group when adding it, instead of saying Any. This will prevent the clash on localhost port 25.
Option 2: Disable sendmail completely
You can run the following commands to ensure it does not start at boot
service sendmail stop
systemctl stop sendmail
chmod 000 /usr/sbin/sendmail