Hi;
As my understanding you can't access the https://mail.server (which is behind firewall - inside zone) from you private network 172.24.1.0 (which connected to Router 3rd interface - Firewall outside zone).
After review the configuration the missing nat statement for 172.24.1.0 network on firewall and also verify firewall policy (outside interface) allowed 172.24.1.0 network to communicate mail server.
Thanks & best regards;