Start new conversations from
If you have more than one email address or phone number listed under “You can be reached for messages at,” choose the email address or phone number that you want to use for starting new conversations. When you start a new conversation with someone, the person receives messages sent from the email address or phone number you choose.
When someone starts a conversation with you, you send messages using the email address or phone number the person used to start the conversation.
Blocked
Review addresses or phone numbers of people that are blocked. See iPhone Manuals website