I'm not sure whether this is the right forum for this. 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 I was wondering if anyone here might be willing to lend a hand. 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, I've done a bit of googling and asking around, but my understanding of all this is still very shaky. My response to this would be that we should tell the client to tell their IT dept to whitelist our domain or our IP, because it appears their mail server has blacklisted us. Now, my questions are: 1) Does that sound right to you? Is that what should be done here? 2) From what I gather, there are other possible concerns here: a) blacklisting on a broader level Maybe our domain or our IP has been added to some blacklist and now mail servers across the internet will be rejecting our email. b) 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. Are those two legitimate concerns in the sense that besides telling the client to whitelist us we would need to look into whether there's a problem with either of these two because others may be not getting our mail. Is there anything else besides these two possibilities I should check?