Spam from a known good email address - how to train?

I’m seeing increasing amounts of a certain type of spam. The From is a valid email address from a friend or relative, and there are several addresses CC’d that I don’t know. The body is a single spammy-looking URL. I have no idea how spammers are able to do this, but it does get through SpamSieve’s filter, probably because the From address is someone I’ve written to before.

Should I train these types of emails as spam, or should I just delete them? I’m concerned that if I train them as spam, SpamSieve will blacklist the From address and then legit email from that person won’t get through.

Yes, you should train every spam message that gets through as spam. The blocklist won’t be a problem if you also train a message from that address as good (or you could manually uncheck the blocklist rule to disable it).

If the sender is in your macOS contacts, you may also want to check the preferences to make sure that SpamSieve isn’t always classifying messages from that address as good for that reason.