Server error 554 5.7.1 relay access denied in Outlook

Sending to anyone outside our own domain bounces instantly with 554 5.7.1 relay access denied. Mail inside the domain flows fine.

Recently switched the account from the office PC to my home laptop, same settings copied by hand. What did I break?

Nothing broke, something failed to copy: 554 5.7.1 is the outgoing server refusing to relay, and a server relays for exactly two kinds of clients, those on its trusted network and those who authenticate. The office PC sat on the trusted network and never needed the authentication, so the settings you copied never contained it. From your home connection the server correctly sees an unauthenticated stranger asking it to carry mail elsewhere. Internal mail still works because delivering to its own domain is not relaying.

1

Open File, Account Settings, double click the account and click More Settings.

2

On the Outgoing Server tab tick My outgoing server (SMTP) requires authentication and select Use same settings as my incoming mail server.

3

On the Advanced tab move off port 25 if you are on it: set the outgoing port to 587 with STARTTLS, or 465 with SSL, per your provider's published settings. Home ISPs widely block port 25 anyway, so this step prevents the next mystery too.

4

OK out, restart Outlook and send an external test. The relay refusal disappears the moment the server sees credentials.

If authentication is already ticked and correct, the rarer variants: the provider requiring the full email address as the username where the office setup used a short form, or the outgoing password having drifted from the incoming one after a change. Both surface in the same More Settings dialog, and your provider's SMTP settings page is the authority to check the values against.

The authentication tick was indeed missing, the office network had been vouching for me all along without my knowledge. Ticked, port 587, external mail flowing from home.