In general, hardening is the process of improving security of a system in order to lessen vulnerabilities. Given that email has been around for almost 40 years and was never meant to be a secure communication medium, we must recognise that it is not and will never be totally secure. As a result, we use the phrase hardening rather than securing.
For email to remain useful, two-way communications must be maintained. The two goals of these changes are, therefore:
To have your email read, it must get to your recipient and appear in their Inbox. It can't be blocked along the way or delivered to their Junk/SPAM folder.
But for this to happen, we must ensure tighter controls on your domain. This prevents unauthorised Internet users from using your domain and provides a recipient with as much information as possible about the validity of your email. This information offers reassurance that you did actually send the mail.
Despite an increase in the practices above, there will always be unsolicited email (SPAM) and malware (viruses) attempting to get seen. Therefore, filtering this email from your Inbox is important too.
To do this, we use the same information (discussed above) on the email we receive, to determine how likely the email you receive is legitimate.
The filtering process is enabled on each mailbox with a normal set of rules. This is an ideal staring point. But each individual has their own requirements; an email that is "clearly spam" to you, will be viewed as legitimate to someone else.
So the system sends an "Email Quaratine Advice" with what it has stopped each day. These emails are a legtimate part of the system.
Firstly, they allow you to recover emails that may have been wrongly classified. But in the process, releasing a message also trains the system not to mark similar email in future.
These indicate the messages held, by default, in the last 24 hours. If there is a message you wish to receive, click the 'Release to Inbox' on the right and follow the prompts from there.
If there is no legitimate email, you can simply delete the advice. These messages will be automatically removed after 14 days. They do not contribute to your email quota while in quarantine.