You can configure Fastmail to do that, but I guess it would be in reverse. I have some old Google accounts (old student email and pre-custom domain gmail) confi

You can configure Fastmail to do that, but I guess it would be in reverse. I have some old Google accounts (old student email and pre-custom domain gmail) configured to forward mail to Fastmail, and then Fastmail configured to send email via Google for those email addresses too. https://www.fastmail.help/hc/en-us/articles/1500000278022-Using-other-email-addresses https://www.fastmail.help/hc/en-us/articles/1500000278022-Us...