I guess, this is an easy one, however, I can't figure it out.

In Thunderbird and from LAN, I just enter 10.0.0.5 as my IMAP mailserver address and it works without problems, but when I try to connect from outside using my DynDNS.com-address
myName.DynDNS.com
I cannot connect. Therefor my question:
What should I fill in as the IMAP mailserver name to configure Thunderbird as my mail client?
Thanks a lot
dan_bubba