In Gmail, sending email through Fastmail's SMTP modifies From: header

Question

In Gmail's settings, under "Accounts and Import", I added a new email address under the setting "Send mail as". This email address using my own domain, [email protected] let's say. I've configured Gmail to send using Fastmail's SMTP servers.

This domain has been set up on Fastmail. The MX records have been set, and I've added the alias, instructing Fastmail to forward all email to my @gmail.com address.

Receiving mail at [email protected] does work as expected, the email shows up in my Gmail inbox. Sending from Gmail's web interface as [email protected] also works. However, the recipient gets the email as it came from my @fastmail.com email address. How do I fix that?

Answer 1

There is a setting in Fastmail that you need to change. Open the settings, then "Identities & Fetch" under "Mail", and find the checkbox that says:

Force all emails I send through 3rd party email clients to come from this address.

Make sure this checkbox is not ticked.