Many lost emails with my own SMTP-server


I've set up my own SMTP-server because my ISP sucks. I can't use their SMTP-servers with my university's email(that doesn't provide SMTP-servers outside of their net) somehow..

I used postfix enabler on MacOS 10.3.9 on a beige G3 with xpostfacto. It has worked great so far.. that's what I thought. But it seems that some of the emails I send doesn't arrive.
How is that? The postfix-log says that the email was sent.
I'm on a ADSL-connection, and I have a domain-name.

I realize this is the MacOSX-server forum, but I thought that I probably would get the most help from here
What kind of emails are they? RTF? Plain-text? Attachments... what kind?

It could be that the recipient has some sort of junk mail filter, or their ISP does and is incorrectly flagging your emails.
Standard RTF-mails from No attachments.
Only personal emails to friends and stuff like that.

I've had lot's of problems with hotmail-adresses. I even set up an own hotmail-adress to test and send mail to. The first mail was identified as junk-mail. Then I added the adress to hotmails safe-list and tried to send more emails. Now I've sent 3 emails and none of them made it through. Not even as junkmails!

This is really disturbing..
If it's solely with hotmail, then I would think that hotmail is rejecting your emails because it doesn't recognize your computer as a "known good SMTP server." It's probably checking what SMTP server sent the email against a list of known, good, non-spam SMTP servers and deciding that since it doesn't know about your SMTP server that it's going to flag the messages as junk.
Yeah, I figured it could be something like that. But why would it let through my first mail? And some other friends I have that have hotmail-accounts have gotten all the mails I sent them.

And it's not only with hotmail. I remember now that some of the mails I sent to a friend at the university also got lost..