Sent emails are marked blocked

Recently when I send an email and then check in sent messages, it's showing on all sent emails a red circle with line thru it beside recipient. there's also a header message "This message is from a sender in your blocked list." The sender is receiving the message. This has just started the last week

Why is this occurring?

prior responses say to go to mail on your phone and unblock yourself. With current software i cannot find a place to even see this info. Anyone know how to undo this. The latest update of the phone has so so many problems!