ifconfig output:
sit0: flags=193<UP,RUNNING,NOARP>  mtu 1480
        inet6 ::127.0.0.1  prefixlen 96  scopeid 0x90<compat,host>
        inet6 ::192.168.1.6  prefixlen 96  scopeid 0x80<compat,global>
        inet6 ::192.168.0.244  prefixlen 96  scopeid 0x80<compat,global>
        sit  txqueuelen 1000  (IPv6-in-IPv4)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

sit1: flags=209<UP,POINTOPOINT,RUNNING,NOARP>  mtu 1480
        inet6 2001:470:6840::2  prefixlen 48  scopeid 0x0<global>
        inet6 2001:470:1f09:7e::1  prefixlen 64  scopeid 0x0<global>
        inet6 2001:470:6840::9  prefixlen 48  scopeid 0x0<global>
        inet6 2001:470:6840::6  prefixlen 48  scopeid 0x0<global>
        inet6 2001:470:6840::10  prefixlen 48  scopeid 0x0<global>
        inet6 2001:470:1f09:7e::4  prefixlen 64  scopeid 0x0<global>
        inet6 2001:470:1f09:7e::9  prefixlen 64  scopeid 0x0<global>
        inet6 2001:470:6840::1  prefixlen 48  scopeid 0x0<global>
        inet6 2001:470:6840::8  prefixlen 48  scopeid 0x0<global>
        inet6 2001:470:6840::5  prefixlen 48  scopeid 0x0<global>
        inet6 2001:470:1f09:7e::7  prefixlen 64  scopeid 0x0<global>
        inet6 2001:470:1f09:7e::8  prefixlen 64  scopeid 0x0<global>
        inet6 2001:470:1f09:7e::3  prefixlen 64  scopeid 0x0<global>
        inet6 2001:470:1f08:7e::2  prefixlen 64  scopeid 0x0<global>
        inet6 fe80::c0a8:106  prefixlen 64  scopeid 0x20<link>
        inet6 2001:470:6840::4  prefixlen 48  scopeid 0x0<global>
        inet6 2001:470:1f09:7e::10  prefixlen 64  scopeid 0x0<global>
        inet6 2001:470:6840::3  prefixlen 48  scopeid 0x0<global>
        inet6 2001:470:1f09:7e::6  prefixlen 64  scopeid 0x0<global>
        inet6 fe80::c0a8:f4  prefixlen 64  scopeid 0x20<link>
        inet6 2001:470:6840::7  prefixlen 48  scopeid 0x0<global>
        inet6 2001:470:1f09:7e::2  prefixlen 64  scopeid 0x0<global>
        inet6 2001:470:1f09:7e::5  prefixlen 64  scopeid 0x0<global>
        sit  txqueuelen 1000  (IPv6-in-IPv4)
        RX packets 878  bytes 77840 (76.0 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 3186  bytes 286878 (280.1 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

/etc/postfix/main.cf:
inet_protocols = ipv6
inet_interfaces = all

smtpd_banner = $myhostname ESMTP $mail_name (Debian)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = born2host.com
alias_maps = hash:/etc/aliases,hash:/var/lib/mailman/data/aliases
alias_database = hash:/etc/aliases
mydomain = born2host.com
myorigin = mail.born2host.com
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain,
relayhost =
mynetworks = 127.0.0.0/8 192.168.0.244/28 192.168.1.6/28 [2001:470:1f09:7e::8]/64 [2001:470:6840::8]/48 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
relay_domains = born2host.com, mail.born2host.com, srv.born2host.com, mx.born2host.com, lists.born2host.com,


I skipped some lines from main.cf as they are default by installation. As I`m using NowTV (UK) as an internet provider I think that their router is blocking me. But the server is DMZ + I added firewall rules in the router to accept any requests on port 25. As they provide me with IPv6 the router can`t be configured with the tunnel. I think this is the problem with the Now TV Hub Two.

It seems it just works perfect with bridge modes. I am trying with the manual static & Slaac but failing.

Well at each point you turn off bridging you're essentially cutting off and creating a separate network so you will have to manually configure at least one thing that can actually route ipv6 (respond to rs that is)

How exactly are you configuring the devices manually? Also is there way to get into the settings anything other than the web based one because it was completely useless at least to me.

What do your routes/postfix configuration look like?

Everything gets addresses fine.  Traceroute/ping gets me a "connect: Network is unreachable."  All other connection attempts through the interface just hang.  Additionally, I get the same results if I disable the firewall completely, allowing all traffic from all sources.

If you can ping HE over IPv6, that means the tunnel is up and the issue is on your side.

What isn't working? Do your clients get IPv6 addresses? Where does traceroute break?  You haven't provided many details

The server on HE's end of the tunnel.

When you say gateway server, what do you mean by that?

I can ping the gateway server on both v4 and v6.  Neither my public v4 nor my public v6 (I'm on a VPS provider that gave me both) has changed.

You can ping the gateway server over Ipv4?  Any chance your public IPv4 address changed?

About half a week ago, my tunnel (that had been functioning perfectly for a month or so) suddenly stopped working.  In trying to fix it, I've determined that I can ping the gateway server just fine, but nothing else.  Additionally, the portscan (of the client IP) provided by https://tunnelbroker.net/portscan.php is very wrong.  It fails unless I specify -Pn, and lists the following ports as open (none of which are open in my firewall config):

6666/tcp
6667/tcp
6668/tcp
6669/tcp
7000/tcp
9999/tcp

Normally I would assume that I had grossly misconfigured something, except for the fact that it was working perfectly and stopped working seemingly without provocation (no reboots, network outages, etc.).  Is there someone that I should contact about this?