Check the spelling of the username and domain in the destination email address. Many servers on the Internet are UNIX-based servers which are case sensitive. On these servers, UserName@twc.com, for example, is not the same as USERNAME@twc.com or firstname.lastname@example.org.
It is also possible that your recipient has a “full mailbox,” which means their provider’s system has reached its mail storage limit. You can also verify if the recipient has blocked your email address or domain.