I have a Macbook Pro and I’m using the Apple email service. Recently when I send emails I often get this message back.
“I’m sorry to have to inform you that your message could not
be delivered to one or more recipients.
<firstname.lastname@example.org>: host mx.centurylink.net[18.104.22.168] said:
554 5.7.1 [P4] Message blocked due to spam content in the message. (in reply to end of DATA command)”