Roger, if you are using a bad hostname for your mail server, things like this happen:
NOQUEUE: reject: RCPT from unknown[xx.xx.xx.xx]: 504 5.5.2 <hyfjfrwsdfgka02421>: Helo command rejected: need fully-qualified hostname;
Well, you would like your mail server to receive mail from hosts like hyfjfrwsdfgka02421 instead of, for example, stmp-008.serious-domain.com?
Most spammers are doing the same thing, but I'll assume that you're not a spammer (although 70% of all spams received on my server is coming from spammers who use iRedMail themselves , probably because it's too easy to install).
If you really want your mail to get accepted by serious mail servers, you need to setup a domain for it, register your mail server as MX, put some SPF and DKIM records and so on.
And especially, add the proper hostname in all configuration files, like postfix, amavis etc. (iRedMail is already doing this, so I don't know why yours isn't working).