Internet mail protocol standards dictate precisely what the format of each sender address can be, and our service will reject any messages sent that do not meet these standards.
Typically, when users receive this error it's due to a syntax error in the sender's email address (such as omitting the @ symbol). You will need to correct this error before you are able to send any messages. Please ensure that the sender meets the standards established by RFC5322. You can also read a simplified description of the proper syntax on Wikipedia.
If you have verified your sender address, believe it is completely valid, and are still receiving an issue, please open a ticket within our Support Center and we will look into the issue for you.