Error Code Explanation 5.5.2

550 5.5.2 Invalid address.

One of the "To:" (or Cc: or Bcc:) addresses in your email had improper syntax. The proper syntax for the To: line is;

To: Jane Doe <jdoe@example.com>
An acceptable alternative is to just list the address without the name or the angle brackets like so;
To: jdoe@example.com
Lists of address should be separated with comma (','). If the name has a comma, then it should be in quotes.
Bcc: "Doe, Jane" <jdoe@example.com>, "Doe, Bob" <bdoe@example.com>
or
Bcc: jdoe@example.com, bdoe@example.com
Most email clients accept the archaic form;
To: jdoe@example.com (Jane Doe)
But it should be avoided.

Common ways for this error to occur are;