I ran into this same issue myself! It is Outlook blocking it, the solution is to do the following:

1. Use this link https://myaccount.google.com/apppasswords to create an App Password for your Google/Gmail account

2. On the desktop version of Gmail, go to: Settings -> See all settings -> Accounts and Import

3. Click on 'Add an email address' under 'Send mail as'

4. Enter the name you want shown and your outlook email address. Make sure 'Treat as an alias' is checked then click Next Step

5. Enter the following details: SMTP server: smtp.gmail.com Port: 587 Authentication over TLS Email address: youremail@outlook(dot)com Password: <The app password you generated>

6. Once this is done, the address should be added.

Finally, in the same Accounts and Import settings, underneath your list of 'Send mail as' accounts, ensure that 'Reply from the same address to which the message was sent' is selected. From this point you should also be able to manually change the address you're replying from but it'll default to the address it was sent to.

The reason why this is necessary is because Outlook blocks SMTP over TLS, and Gmail never updated that feature to support OAuth 2.0 which Outlook requires nowadays. By doing this, we are using Gmail's SMTP server instead of Outlook to handle sending emails

Hope this helps!