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.
<____@centurylink.net>: host mx.centurylink.net[206.152.134.65] said:
554 5.7.1 [P4] Message blocked due to spam content in the message. (in reply to end of DATA command)”
—–
David Korte
So the problem most likely lies on the other end. The person you are sending the message to is using ISP email, which is the rock-bottom of email services. Hopefully they will soon move to a better email service like iCloud or Gmail. See https://macmost.com/five-reasons-not-to-use-your-isps-email-service.html
As for as what you can do, there's not much. However, perhaps you've got a lot going on in your message? Does your message have a lot of text, lots of quoted text, or maybe images? Are you using a complex signature, maybe with a quote or an image?
You could try sending that person a very simple message to see if it gets through. Start a message and put nothing in the body but a simple piece of text and maybe your name. Then choose Format, Make Plain Text to make your message just text and nothing fancy. Then see if that gets through.
If it does, then you know that the error you were getting (spam content) was at least being triggered by something in your original message. Maybe look at it carefully and see what you think could be the culprit.
Otherwise, all you can do is inform the recipient that they aren't getting their email because their email provider is blocking legitimate email. There's probably nothing they can do since ISP email services don't provide any support. They should just move to a better email service.