CXSNDR error

I received a CXSNDR error trying to send an e-mail to privacy@usps.gov.  Are you telling me that the U.S. Postal Service .gov domain is not supported???

I was sending the message using Mac Mail from my home network. There were no attachments. All other e-mail is working normally. 

Walter

By the way, I have a screen shot of the error that I could not attach to this problem report. The Insert function only accepts web URLs or a Community (Search), whatever that is. I could not drag & drop a .png or .pdf from my hard drive.