I have a Macbook Pro and use the Apple email service. Recently, when I send email, I often get this message back.
“I'm sorry I had to inform you that your message could not be delivered to one or more recipients.
<____>: host mx .centurylink.net [18.104.22.168] said:
554 5.7.1 [P4] Message is blocked due to spam content in the message. (in response to the end of the DATA command) ”