Q: When someone tries to send me an email they get a bounce back saying they are on a blacklist, i.e. “550 JunkMail rejected”, “550 in an RBL, see Sender has sent to LashBack Unsubscribe Probe accounts”, etc.

A: The issue is that the sender has been "black listed" as a spammer. Their IP address  is on a Real-time Block List (RBL) due to spam sent from their server.  The user that is trying to send email to your server needs to contact their ISP or hosting provider for support on getting their Domain Name or IP Address removed from the RBL(s) in question. You can check the sender's server IPs here: http://mxtoolbox.com/blacklists.aspx.

In the meantime, we can attempt to add the sender's server IPs to our server's whitelists.  To do that, we need to receive from you the full error message and from/to emails involved in any issue, including the sender's server IPs - the actual email forward or attachment is best.