imap reachable for vpn ip 10.9.0.101 ? was once possible, but I cannot remember

[ATT1]

Hello,
Sorry again for a somehwhat foolish post, but I need a little help again.
My Zentyal 7 Server is also reachable for VPN users with the VPN ip "10.9.0.101". SSH works, etc.etc. -- so far so good.
For my "old" Zentyal 7 Server, imap also worked : I used the IMAP settings : Server 10.9.0.101, port 143, STARTTLS, password normal,
and the user could read his e-mails over VPN -- very convenient!
However I cannot remember what I did when I configured the IMAP like that.
Now with the newly installed Zentyal 7 Server I also want IMAP to be reachable from the VPN ip "10.9.0.x" .
What do I need to change inside the system so it works?
Or is "Thunderbird" the culprit and only certain Thunderbird versions will accept this ?

Any help greatly appreciated.

[ATT1]

Ah, it was the firewall, it needed some additional rules, now it works.

[ATT1]

It is also noteworthy : If you have a fresh installed Zentyal replacing an old Zentyal with the same server name and domain name as before,
then you need to add the Thunderbird profile again by making new profiles with "thunderbird -p" ; the old profile data is going to mess with the "new" server that has the same IP and same name and same domain name as the old server.