If you try to send invoices in BUSY, you get an error saying invalid email address. Even though the address is correct, you have to check the below reasons:
1. While sending email, make sure you have a valid email address with the correct email ID.
2. Also, make sure if you are sending at one time for two or more email ID's, you need to put this in between the ID's.
3. Also, make sure there is no blank email ID or wrong email ID.
4. Also, please check whether in two or more email ID's there are no special characters like / or, or any other symbols.