A message sent to a third-party server bounces back with an error message containing "550 Message blocked."
"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.
The following article has general guidance on preventing your mail from being treated like Spam:
How to Keep Your Email Out of the Spam Folder
Things to Check
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:
For further reading, see: Improving Email Deliverability.
Everything Checks Out
What else can you do? If your server isn't sending spam, your IP isn't on a blacklist, your email is properly authenticated, and you're still getting this bounceback, it's time for you to reach out to the third party email provider and ask them for further assistance as they should be able to provide more context as to why the issue is occurring.
Here are a few tips for identifying the remote mail server’s technical point of contact:
You can look up the email hosting provider based on the mailserver hostname or IP address in the bounceback or log, but the most effective approach is to reach out to the recipient (through an alternate contact method if needed), and asking that person to contact their hosting company or email provider.