Can't send email to Gmail

I have setup Cyberpanel around a month ago. Everything seems to be okay with the configurations. I get a 10/10 score on both Email Delivery Tester by CyberPanel and Things seem to be okay on mxtoolbox also. You may check the screenshots below. But still, the emails aren’t sending to Gmail. It returns an error whenever I try to send email to a gmail account.

This is the error that I am receiving while sending email to Gmail:

There seems to be a problem with SMTP Relay. Can you please tell me how I can fix this issue?
Looking forward to your reply.
Thanks in advance.

Is port 25 open? If it is not open, contact the server administrator.