How to avoid bouncing or blacklisting

How to avoid your mail being branded as spam while it isn't.

Sysadmin

Things your (ISP's) system administrator(s) should take care of.

DNS

Make sure you have a properly functioning reverse lookup. This should match the forward and may not be an alias.
If your system is dual stack (both IPv4 and IPv6), make sure this works for both IP addresses.

Mailserver

Time

Your servers should use NTP to synchronise to an atomic clock. Mail with incorrect time is often rejected.

Whois

Make sure both your domain and your ip nr(s) are properly registered with whois. Check that whois actually works properly. Some TLDs (Top Level Domains) don't have a whois at all. Do not use these domains.
Sending mail without any form of registration is a spammer's heaven. You don't want to support or be associated with spammers, do you now?

User

Things you should take care of.

ISP

Make sure the stuff your ISP does is actually RFC compliant. If not, change ISP. If in doubt, get expert advice. And don't forget to get a 2nd and 3rd opinion.

Mail