[opensuse] DKIM question

Hi, A while ago there was a post about e-mail configuration and this mentioned DKIM. I googled for it and I was interested. I want to configure my mail-server to use it to reject forged sender-addresses, but I think I can't. I have users that mail from outside of the company. They could use our mail server, so DKIM would work. But some providers totally block port 25 so these users are forced to use the mail-server from that provider. Since this has no clue about my server's DKIM I fear that my server will reject messages to internal recipients. Am I right ? Or am I too concerned ? I could configure postfix to also listen on a high port, but I don't like this. TIA. -- Met vriendelijke groeten, Koenraad Lelong