Testing your forwarding from the same Gmail account
If you're using Gmail or Google Workspace, and you try sending a test email from your own address to your ImprovMX alias that forwards back to yourself, it often ends up in the spam folder.
What you're seeing is Gmail's (and Google Workspace's) self-loop detection — not a problem with ImprovMX.
Why this happens
If you send an email from richard@gmail.com to richard@piedpiper.com (managed by ImprovMX) and richard@piedpiper.com forwards back
to richard@gmail.com, Google sees the forwarded copy arrive
with the same Message-Id it just sent out and silently deduplicates
it. You never see the message — even though ImprovMX delivered it correctly.
The same applies to Google Workspace custom domains, i.e. richard@piedpiper.com as a Google Workspace address.
What ImprovMX does about it
When ImprovMX detects a self-loop to a Google-hosted destination, we
rewrite the Message-Id header and re-sign the message with our
own improvmx-mails.com DKIM key before delivery. This lets Google
treat the forwarded copy as a new, distinct message and show it in your inbox
instead of deduping it away.
A side effect is that the delivered copy often ends up in the spam folder due to broken DMARC and the new DKIM signature. This only applies to self-loop test messages — incoming mail from other senders is delivered untouched. See Is improvmx-mails.com a legitimate domain? for more on where that domain shows up in headers.
Recommended: test from a different address
To test your forwarding cleanly without tripping Google's loop detection, send from an address that is not your own gmail address. Alternatively, sendtestemail.com is a free service that sends a test email for you.