0x800ccc6a error 451

Below is an issue I'm getting with a clients E-mail and this seems to only be at their home. No matter what mail client you want to use or PC you can't send mail BUT it will recieve mail. It will see the incoming and outgoing servers but can't send mail. I tried to ping the server and it was found and I did a telnet to the server and with port 25 and 26 since they use 26 in some cases at their host and that responded too. They use Verizon and this issue happened right after verizon got shut off then reconnected. They are NOT using verizon for their mail they are using another service to host their mail. I cannot figure out what would be the cause of this if anyone has any suggestions of what to look at I'd be greatful.

Outlook Error Message:
Task 'mail.*************.com - Sending and Receiving' reported error (0x800CCC6A): 'Your outgoing (SMTP) e-mail server has reported an internal error. If you continue to receive this message, contat your server administrator or Internet service provider (ISP). The server responded: 451 Please try again later.'

Outlook Express Error Message:
An unknown error has occurred. Account: 'mail.*************.com', Server: 'mail.*************.com', Protocol: SMTP, Server Response: '451 Please try again later.', Port: 25, Secure(SSL): No, Server Error: 451, Error Number: 0x800CCC6A

Thunderbird Error Message:
"An error occurred sending email: The mail server sent an incorrect greeting: The Server has reached its limit for processing requests from your host. Please try again later.."

Error 451 indicates that the mail server is busy. If it has been busy for a significant period - more than a day, then contact the ISP.

Thanks for the input NooNoo but here's a strange thing. I can test it from my house and it works. I even called them and told them it works and had them test it over the phone and all computers won't send in their house. Even though I test at the same time and go through.

It may be that the IP in use is blacklisted....

Interesting. I assume if it's blacklisted that would still allow me to ping and telnet it from their location. I'll see if she can talk to her ISP and have them give her a new IP.

It is the mail server that has identified the IP as a bad I.P. Telnet and ping do not interface with the spam prevention software.... Hence why I said to phone the ISP....

That's what I inferred, it makes sense that it would allow that but wouldn't let mail to be sent.