Ipsec key problem

[decibel83]

Hi.

I'm configuring an Ipsec VPN between two Pfsense systems.
The server has a static IP address, and the client has a dynamic IP address and it is natted.
The connection works only if I setup the real client's IP address as pre-shared key's identifier, but if I insert an e-mail address or a FQDN (like foobar.local) it doesn't working why the server doesn't recognise the key.
Has the identifier to be a valid hostname? Can I invent an identifier or it has to be a real one?

Thank you very much!
Goodbye.