Strange issue: 550 Authentication is required for relay

I have a client who has a couple of domains setup that are catch-alls (e.g. client@theircatchall.com) that relay to a single email account on my SM server (e.g. client@theirsmaccount.com).

If I send to the catch-all account from a Gmail account, the email comes through fine, no errors.

If I send from an account on the same server as the receiving email account or there have been a few people outside of the server that has had the same error, I receive:

<client@theirsmaccount.com> (expanded from <client@theircatchall.com>): host
mail.theirsmaccount.com[***.***.***.***] said: 550 Authentication is
required for relay (in reply to MAIL FROM command)

The account I'm sending from is authenticated and works fine.

Any idea why this would be, it has me stumped at the minute?

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