able to receive email but cannot send

Hello
i created a user account for a domain and then created an email account under that domain.
I am able to receive emails to this acccount but not able to send from it.
I get this error message when i try to send.
SMTP Error (550): Failed to add receipient "someuser@domain.com". Recipient address rejected: User unknown in virtual mailbox table.
What could be the issue

Has the user been added to the Mysql database table that Postfix uses under mailbox ?

I really wish people would stop changing things when they ask questions.  There is absolutely NO NEED to "Mask" domain names, "Public IP's" or "Certificate Info that the browser displays".  When you do this, you make it almost impossible to help you.

Please DO mask usernames/passwords however.

END OF RANT

now onto your question (Cause now I'm guessing)

"Somebody@domain.com":
domain.com hosted on your server:  Glenn is correct
domain.com is NOT hosted on your server.  See below

SMTP Error (550):  https://serversmtp.com/smtp-error/

550

It usually defines a non-existent email address on the remote side.

Though it can be returned also by the recipient’s firewall (or when the incoming server is down), the great majority of errors 550 simply tell that the recipient email address doesn’t exist. You should contact the recipient otherwise and get the right address.