I agree with @terribleplan@lemmy.nrd.li that the inclusion of a postfix SMTP container and using that in the example configs is probably doing more harm than good.

You shouldn't need the Postfix container at all unless you want to manually set that up and know what you're doing. You configure your SMTP settings in the config.hjson file as outlined in the example defaults.hjson file. I'm using a third-party SMTP provider to avoid any deliverability issues with sending emails directly from my server's IP address. Just ensure you configured your DNS correctly (DKIM, etc.), punch in the info, and it should work.

This sounds like a mail delivery problem rather than specifically a Lemmy problem. The Lemmy server configuration tells your instance how to send email; the default is to use an SMTP server on localhost.

Assuming you have a mail server running, try checking its logs. Maybe Gmail is rejecting mail from your IP address, or maybe your hosting provider or ISP is blocking port 25 by default as an anti-spam measure?

I got an email when I signed up as a test. It is most likely spam filters and/or improper email server setup. I will try to help debug in a bit.

Important info for instance owners to get right, thank you for posting.