[mths]

> go into fastmail settings and create a "sender identity"

Not anymore, they've made it much simpler for you now by combining "aliases" and "sending identities" into "my email addresses". If you thought it was confusing before when you had to click "create new" under "sender identities" in settings, now all you need to do is:

1. Head to Settings → Migration → Import page.

2. Click on Add external sending address (SMTP) without importing emails.

3. Enter the email address and click Next.

4. On the following screen, skip the password prompt and click on Manually configure.

5. Disable the option - User authenticated SMTP when sending.

6. Save the changes.

And just like that you can start sending emails with your new address, easy peasy.

[kazinator]

What you're describing sounds hokey.

A "Sender identity" should just be a different From: mail header; it shouldn't involve disabling SMTP authentication. The SMTP envelope address and authentication are unaffected.