You said on reboot. Everything restarts on reboot and order is important. You can get them sent because there is not a firewall that comes later.
kerbinet script runs in a continuous loop sa no need for crontab. All relevant script are done in that way. I am not sure but this is what I noticed.
Also try to isolate.. so first send an email using telnet if ok then using smtp-cli
Also you didn’t specify if you server is internal or external so try to telnet and smtp-cli on both..
If everything works then there is something else in scripts. Scripts are on /root/kerbynet/scripts