What would this error message mean?

[transparant]

I keep trying to send out an email (a very important one too) and I keep getting this message after I send it:

destination not valid within
DNS

I've emailed this contact before without any problems. What could this mean?

 

[bicker]

It could be a temporary issue, but the error message means that according to the mail server's data, the domain you're sending to is not valid.