if your zimbraMailMode is http then i'm not sure how you are able to connect with https

. you can enable ssl access by running "zmtlsctl mixed; tomcat restart"
so the software firewall *is* enabled but it looks like it should accept all packets
what happens when you try this from the client machine (replace SERVER-NAME with the zimbra server hostname):
telnet SERVER-NAME 80
telnet SERVER-NAME 443