You really need to go back and read and understand Charlies response.
He is a world class expert on this mail system (I think he probably helped write at least some of it) - which is more than can probably said for the people at Spiceworks.
Your server is doing exactly what it is meant to and is replying as instructed by THEIR setup.
Charlie may add to this thread, but I am pretty sure he'll echo something similar.
The plomvakc.on.spiceworks.com address is working as I am able to email directly to it from my own email account.
This is nonsense and means nothing - it is comparing apples and pears
The main problem is that the email address is being rewritten. When it rewrites the email then goes to an address that is just used for pointing to the MX record rather than being an active email address.
It is not rewriting. Just replying correctly, as it is meant to do by published RFCs. If they publish the wrong information it isn't your mail servers fault.
Unfortunately, this isn't something on our end but rather the sending email itself. There isn't really anything else we can do on this side beyond saying the system shouldn't be rewriting the address as it comes in. That is a function of some email systems with CName expansion which is why we mention it needs to be turned off.
AFAIAA your server isn't doing CNAME expansion...... nor is it rewriting. This comment just means they can't be arsed to try and fix their own mess.
I know it doesn't necessarily help you, and this is always the problem when you are up against a large company. They just can't be bothered to change, or don't know who to contact in their organisation to get it fixed so just lob the issue back at you.
I am not sure exactly how you CAN bodge your mail server to try and conform to their messy systems, and personally even if I knew it would be unprofessional to suggest it.
You really need to go back to them armed with the right information from here and tell them where they are wrong. It might do you some good to go and read up on some of the whys and wherefores of DNS, MX records etc so you fully understand what is going on, and why they are wrong. Charlie may be able to give you some pointers on how to do that, but reading yourself will help.