“550 Message blocked” is a generic error from several email providers, and it can have different causes. Usually, this will be some a spam filter of sorts. Sometimes, the bounce will provide a URL with an explanation explaining the issue and what to do.
It’s difficult to determine the specific reason why a remote mail server blocks emails from your server or marks them as SPAM. The remote servers use their own custom guidelines to make that determination.
You should ensure that your domain has valid SPF, DKIM, and PTR records. These can be checked in Email Deliverability in cPanel. If the interface says, they’re valid, great. Otherwise, fix them.
A third-party provider might blacklist the server IP. You may want to look into seeing if your IP is listed at one of these providers: