"smpt;550 5.6.0. Sorry, looks like SPAM to me"


I'm applying for a tech support job and I got a test that I have a week to complete, so I assume they expect me to do some research for it, so, here I am :)

I'm supposed to give my response to a support ticket. The scenario is that a client is not receiving email alerts we send out and the mail delivery report says "smpt;550 5.6.0. Sorry, looks like SPAM to me"

Now, presumably, the client's mail server has blacklisted our IP or domain and we need to tell the client to whitelist it, would that be a correct assumption?

My question is, is there something else I ought to worry about?

1) SPF issues

Elsewhere, I've been advised that this may be due to SPF issues. I barely understand what SPF is, but with some googling, I've gotten a vague image.

Is this something I should be worrying about in this scenario? That our SPF is messed up?

Wouldn't that result in ALL our emails being rejected as spam, not just the ones sent to this client?

2) global blaclisting

I'm not sure how blacklisting works, but if their mail server has blacklisted either our IP or domain, should I be worried that we might be blacklisted on the level of the general internet or at least many mail serves?

3) anything else?