Error 550 on an address in Eudora

I have added an address to my Eudora address book, and I get this message when I try to send mail: Can't send to. The server gives this reason: 550 5 1 1 Not our Customer." Now when the person's address was sent to me, as a part of a piece of mail to that person and myself, I "copied and pasted" the address into the address book. I thought I had it wrong when my "test mail" did not go, so I asked for it again. I had it correct. I use Eudora, and have not had this trouble with any other addresses. Today I sent mail, got the message. I went to Webmail and used the address, and the mail went through. Is there something in Eudora I should check? Last night I was able to send a piece of mail with this address as a part of it, and it went through OK. But today, no go. Thanks for any help you can give.

Does it happen with just that one particular address?

To answer the question, yes. However, I think somehow the address in the book was corrupt. I found that if I typed the address into the e-mail, or if I replied to an e-mail from that address, the mail went through. I decided to delete the address and re-enter it, and from then on I have had no trouble. I have no idea what would have happened to make it bad, as it was letter by letter the same, but something did it. Thanks so much for replying.

I'm glad, you fixed it :)