Guys at the other server were rejecting mails from all reverse dns that contained "dsl" in their name. And because my domain is something like "madslack.com" (read it "mad slack" - this is an example) they were rejecting any mail from it. I would never expect anything like this.

Thanks for your support, you should close this now.

I can't find out exactly what is the problem, but I know 2 things:
1) Those guys are paranoid.
2) They don't speak to anybody.

So, I guess I can't find out exactly what is the problem, but I'll try figuring out myself.

What's rule #1?

Today I have found out I have a problem with my main server. People trying to send mail to someone else get rejected with a bounce message:

I think this might be because of the fact that my smtp always send EHLO greeting with the main hostname (eg, mx2.xxx.com).

Is there any chance to configure the server to send different EHLO greetings when sending mail, depending from the user's domain?

For example, if user user@aaa-buss.com, and johndoe@tralala.com try to send email through my smtp, then make my smtp send different greetings based on the sender's domain?

Thanks.