As the error message clearly states, the mail is sent perfectly fine, but then discarded by the receiving mailserver because it's been perceived as spam. That is not a problem that could be resolved by a software update.
The mail has a link to a support article. Go to that article and follow the advice there. If that doesn't help, contact the person responsible for the upstream mailserver and work with them to get your host whitelisted.
Nobody here can troubleshoot this for you.