Good morning. I do have an issue, and I would honstly like you guys to hear me out before claiming my stupidity

The situation is involving an email account that is giving 550 relay not permitted (in reply to RCPT TO command) errors to recipients. Here is my test though: I sent an email via my yahoo account to this zimbra account, which is being forwarded to another domain address. Now, I sent my message from yahoo to the user@zimbra.example.net, and i see the message arrived in the zimbra mail box, but about 2 hours later, I received:

This is the mail system at host zimbra.example.net.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

<mary@forwarding.domain.uk> (expanded from
<user@zimbra.example.net>): host
mailcleaner.example.net[192.168.200.30] said: 550 relay not permitted (in
reply to RCPT TO command)
Testing from remote server for mail flow
Now. I see the message sitting in the user's inbox for her zimbra account. the message is being saved in that inbox and forwarded to another email address. So I am suspecting something maybe in the forwarding might be the trouble. She has not received the message from my yahoo account, via the forwarding. Is there somewhere in Zimbra I can check this pathway and configuration? My other thought is, something is not communicating with her new domain very well from zimbra, but we can send her messages straight to the new address, and it works fine. I'm very perplexed and I am still new with administering Zimbra. So I am trying to figure out why the message goes through but still gets a bounce back with relay not permitted. This could be our mailcleaner device filtering the message to her forward.domain.uk address, but keeping the message from my yahoo in the zimbra mail account?

Thank you for your insight and assistance.