Frequently can not receive email or send email

Frequently can not receive email or send email

Hello,
My domain sometimes can not receive email or send email. But, 2 weeks ago seems everything is fine and have no problem. Could you please help me out?
Now when I was making the email test, I saw the message:
"mail.extmail.org gave this error:
Remote server returned not permitted to relay -> 554 5.7.1 < liurenfeng@bio-bench.com>: Relay access denied"
Thanks,