SMTP RCPT TO Invalid address

PROBLEM:
Recently when my web application was sending out SMTP emails out to end users, I noticed the following error-
javax.mail.SendFailedException: Invalid Addresses;
com.sun.mail.smtp.SMTPAddressFailedException: 501 5.1.3 Invalid address
at com.sun.mail.smtp.SMTPTransport.rcptTo(SMTPTransport.java:1196)

CAUSE:
The end user's email address was not correct i.e., instead of being in the format of "user@domain.com", it was formatted as "user" i.e., missing domain information.