"Are there any software available without paying monthly fee's?"
has to take place before the email
goes into the Iridium
system for it to make any difference on your end. Which means you'll need a host service
which compresses any email that hits their server, which means a monthly service fee.
I don't know anyone offering those services but I'm sure there are some out there. The problem still being--you've got to run it on your end as well, to send or decrypt the incoming. And pay for someone's server to encrypt your incoming.
If you--and your penpals--all make a point to use plain text instead of "webmail" and HTML, you will save some 80-90% on file size every time. That's an option in most email clients but it requires human intelligence to select it.