Open relays are a bad thing - they allow anyone to send email from your mail server: it doesn't check that it is authorised to send mail from the mail address on the third party email.
In plain English that means that anyone can send email via your Slice IP from any mail address. This tends to annoy people and your IP will end up on blocklists and your legitimate email will be banned.
So far, we have sent test emails using the command line.
However, there is no real feedback from these clients as to how our mail server responded - did it respond with the correct details? Note that it is possible to send/receive mail with details that are not 100% correct - leading to issues down the line.
Let's look at telnet to check the server responses to mail requests.