Full Guide Based on Frank's Answer
Firstly create a new email account you can use to relay the Firebase emails through the SMTP server with. I personally chose Gmail, but I tested with Outlook and it also works.
You can now find an SMTP server host that will work for your scenario. If you're sending less than 1000 emails per month you can find free and reliable hosts. I chose SMTP2GO
's free option.
Now you've found the SMTP host, add the email address you've chosen as a single sender email
(note that if you do own a domain, you can alternatively use that to send emails).

Note that you will have to verify the email, usually by your host sending a link to the email's inbox. Make sure to check spam.
Once verified, navigate to where you host allows you to add SMTP Users
and add a new user. This will allocate an SMTP username
and password
.

Navigate to the Firebase console, and choose the Authentication
option from the sidebar (within the Build product category).
Go to Templates
→ SMTP Settings
and enter the details of your SMTP server. The username and password fields are to be filled with the SMTP user login you created in the step above.

It is better to use TLS
, but I believe SSL
should work too but it is untested.
Click save, and you're all set up - but there may still be steps to perform depending on your email provider.
Provider Specific Steps
If the emails are being sent to an account managed by Google you will have no issues with your emails being quarantined by anti-spam policies and it will work immediately.
If you are using Outlook, you will have a different problem on your hands. Outlook's built in defender will most likely have auto-quarantined your email under multiple policies - that bit is important.
These policies are likely to be both spam and phish policies. If you unblock one of them, the other will catch it and re-quarantine.
Unblock both policies for the email address, and test. You can see the status of quarantined messages in Microsoft 365 Defender app under Review
→ Quarantine
. Please note that you will need to be an administrator to add global allow policies to your email accounts.
If this still doesn't work it is likely that your company has an additional external filter (as mine did), and you will have to add the IP's manually to the Tenant Allow/Block Lists
spoofed senders tab.