Certificate has a problem

You might follow this step discribing renewal before time

Or just run ./mailinabox/management/ssl_certificates.py -v

And copy the verbose info here:

mail.zaksmail.com: does resolve to that IPV6

Maybe IPV6 is the problem Try disabling IPV6 temporaraly and provision manually.
Disable: wlp1s0.is my network interface yours might be some other name
sudo sysctl -w net.ipv6.conf.wlp1s0.disable_ipv6=1

Provision
Reenable:
sudo sysctl -w net.ipv6.conf.wlp1s0.disable_ipv6=0