What's this? Email Error?

[Bella]

Hey Guys, Hope you can help me out.

Since yesterday (Last night to be precise) I haven't been able to send my friend an email. I use yahoo. The email that went through was sent on Monday, 13 January 2014 12:11 PM and I received a reply to that, but I keep on trying to reply and get this:

Failure Notice

MAILER-DAEMON@yahoo.com
to: me

"Sorry, we were unable to deliver your message to the following address."

.....

Remote host said: 550 Reject: Invalid DNSBL. [RCPT_TO]

She's a mother figure to me and I really need guidance for things right now. Teacher at the school I used to attend. She loves me too much to blacklist me and she wouldn't know how to blacklist someone anyway (not tech inclined)

So what is that error and how do I fix it?

Thanks guys!
I've sent emails before with no problem
Don't think it's my side...Just checked.
@sean Pretty sure yeah, was continuing a conversation and previous email went through
@Sean, I have. Repeatedly (Sigh) and I've checked the email address too. I went to DNSBL and it's apparently a spam website so I may have been blacklisted accidentally. Trouble is, the site's been shut down so I don't know what else I could try?

[Sean]

are you sure you put in the correct email address?

ok then if you are sure try seeing if you can email them again with that same email address if it wont work its probably wrong