Getting an SMTP error message when I try to send an email

Why am I receiving an SMTP* #5.1.0 error when I try to send someone an email?

This error means the address that you are trying to send to does not exist or is inactive. This error could be the result of a typo in the address or the recipient’s account being deactivated/deleted. You may want to check with the person and verify their email address. You can search the address book within webmail, or do a “People” search from the portal.

*SMTP stands for Simple Mail Transfer Protocol, and is a protocol used for email on the Internet. It defines a message format and a procedure to route messages through the Internet from source to destination via email servers.