The user or users have been added successfully, but there was an error in sending the e-mail message. The server may not be set up correctly to send e-mail. To verify that e-mail is configured correctly, contact your server administrator.

The user or users have been added successfully, but there was an error in sending the e-mail message. The server may not be set up correctly to send e-mail. To verify that e-mail is configured correctly, contact your server administrator.

You may come accorss this error while working with MOSS . I came accross this error while adding user to SharePoint group.

You may check outgoing email setting from Central admin , however it's not the issue . As you are already using Sp site and suddenly you face this issue.

To resolve this issue you will have register your server to SMTP server list