two reason this could happened You misspelled the address /recipient doesn't exist at that address. Or you or your server is blacklisted and the recipients server refuses to take emails from you, by declaring recipient doesn't exist the address might be taken of a spammers list.

http://www.theemailadmin.com/2010/05/troubleshooting-exchange-error-5-1-1/

Thanks. I know the first reason is not it as I have sent emails to this individual from my hotmail account. It must be the second reason...they have blocked my email address as spam on their server. I will ask them to look into it and let me know.

Do you get this for all recipients or just for this specific one? If it happens with any other recipient too, you may be connecting to the wrong outgoing server.

Before I call in the troops and wake all server-personal I would take a second look at the address. If copy/paste-ing you can get an extra " or . ) Even if most mailservers accept a mix of upper and lover case Me.MySelf_And_I@NoFool.com some has to have it just their way, Often lover case. And no extra , or ; at the end Also if there is a list of addresses just one have to be corrupt to make the mail bounce.

Also watch for spaces before or after the e-mail address if it's a recipient from your address book, a frequent problem with copy-pasted addresses.