So thats explains why its being rejected. It contains content identified as spam. AS(4810)’ There is something in the content of your emails that is causing the recieving server to block your email when sending via your SMTP server.
This can sometimes happen if you are eg: replying or forwarding an mail from other people where sometimes it has content in it that is suspicious so gets blocked at the receiving end,
It can be something in the header, subject or body of the email or even in an attachment that the receiving server is rejecting due to its detecting spam in those emails. So not an eM Client issue but something within those emails that are causing them to be rejected.