Plesk on Ubunty, port 25 is open, mail service Activated still can't send email.

I'm having an email issue. I got 3 domains on my Vultr server.

Mail Service is activated and port 25 is open but still can only receive and not send. I'm using Vultr's own nameservers.

When sending a mail out I get this:

host[] said: 550-5.7.1 This message does not have authentication information or fails to pass 550-5.7.1 authentication checks. To best protect our users from spam, the 550-5.7.1 message has been blocked. Please visit 550-5.7.1
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