Mail Express v3.3 and later works with IPv6 addresses (in addition to IPv4). The correct IPv6 format for the Administration Login page is:
[protocol][ipv6 address]:[port]
For example:
https://[2001:db8:1234:5678:0:0:0:5]/admin
https://[2001:db8:1234:5678:0:0:0:5]:8080/admin
The square brackets are usually required when writing out a full IPv6 URL, especially when specifying a port other than the default.
Although Mail Express accepts both IPv4 and IPv6 literals in the Hostname box, it is preferable to use something like me.globalscape.com and allow the DNS server to do the routing. Also, you must add the IPv6 port binding on the General Settings page, and the computer running Mail Express Server must have IPv6 set up correctly in the network adapter settings. If end users will access Mail Express Server with an IPv6 address through a proxy, the proxy must be configured to resolve IPv6 addresses.
For more information about IPv6, refer to the Wikipedia article: http://en.wikipedia.org/wiki/IPv6_address.