How to best test from VM with port 25 closed by ISP

[TG Servers]

Hi,

how can I best test postfix delivery from a local VM if port 25 is
blocked by ISP.
My only intention is to setup another VM and make a network between them
and then send mails between them.
Or is there any other solution how I could get postfix from a VM to the
"world"?

Thanks!