I think this is a similar topic we have discussed not long ago. Let me refresh your memory on this about spam email.
What is your definition of a spam email that you have received? Right, most said unwanted email. Wait a minute, how do system knows you want or don’t want the email? Here’s the problem, an email server or an antispam does not know you want that email to be detected as spam.
So, the understanding of an end-user and the definition set in the server are not the same. Well, we try to minimize this false-positive but this is happening.
The less sophisticated antispam uses RBL. RBL is known as a realtime blackhole list, this is the most basic and common way to fight spam. The type of techniques is based on the source of that email. If the RBL has blacklisted the source base on IP address. Somehow it has worked but it isn’t perfect until if someone’s account is compromised and it was used to send spam.
Then with SPF & DKIM, to tell others I only send out my organization’s email on these email servers and signed my outgoing emails as shown in my domain’s DNS. The trouble is others in the organization were not informed if they were sent from anywhere or the server’s IP address is not stated` in the SPF. In this type of scenario, emails were genuine can be rejected or quarantined.
Servers with antispam mechanism, no matter how sophisticated they are, it boiled down to compliance. Servers don’t read your thought, they are set up with preset rules to determine an incoming email is spam or not.
So if you are sending emails from an email server that has not been defined in the DNS or an outgoing email was signed but it was not matched the key published in the DNS. Even that you are representing the organization. your email will be rejected.
More intelligent email antispam read incoming emails if the content or words look spammy, they can be rejected too. These are a few techniques are used in antispam.
I want to mention this to all email server’s administrator, please send a returned message if you reject the senders’ email. I have seen email was rejected and dropped quietly, made tracing difficult.