Postfix issue

[skysel]

I have a following problem:

All outside MTA's which are connecting to my box, have ip 10.0.0.5 (due to firewall and no it cannot be changed). Meaning, I have to add 10.0.0.5 to mynetworks in order for mail to get delivered from my box to some other box. But problem is, spammers can easily take advantage of this so i cannot really have this enabled. I tried enabling smtpd auth but it doesnt work on the mynetworks. So i commented out mynetworks under smtpd_recipient_restrictions and now i get all mails denied..again. I need somekind of solution .. anyone kind enoguh .. please help