When you click the “Send” button to send a message in Yahoo! Mail, Yahoo automatically check the addresses of your intended recipients to make sure that each of them has been entered in a valid format. If Yahoo detect a character or a format which is prohibited by Internet mail standards, you’ll receive a “Problem Found” error on the Compose Mail screen.
Common causes of this error include:
- Missing parts of a domain. Domains must be fully specified. Leaving out part of an address, for example, the “.com,” produces this error.
- “Illegal” characters, including spaces and commas. Because email must be routed through various mail systems, certain characters cannot be included in an address. These include, spaces, commas, colons (:), and a few more. In addition, addresses cannot include more than one “at” (@) symbol. If your correspondent has given you an illegal address with one of these characters, chances are he or she also has a version of the address without them. Spaces aren’t allowed as part of an address, but spaces can exist before or after an address.
- Currently it’s not possible to post directly to a news group through Yahoo! Mail such as news group names like alt.fan.yahoomail.
To send your message, you’ll need to make a change in the “To,” “Cc,” or “Bcc” fields so that all addresses contained in those fields are legal. If you’re using nicknames or group names, make sure that the addresses in your address book are all properly formatted.[source]