The newness of your domain is not the primary problem.
- You have no SPF records on your domain.
- You have no DMARC record.
- You're on the Spamhaus RBL.
the email sent to them to do this always ends up in the spam folder. There’s nothing we can do about that.
There's lots you can do about that.
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Check your MX records. If the email for the domain is actually hosted by Google, verify you implemented the correct MX records. If it's not Google hosted, use the correct MX records for where it is. If you do not have email hosted for this domain, you need to fix that or change the FROM address the validation email to a valid domain/address.
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Create a valid SPF record. This is going to include the SPF records of the email host (i.e. Google, again see their docs) and the ip address(es) of the SMTP server used to send the validation emails. If you're using the bundled postfix container then it's going to be the IP address of your lemmy server.
It looks like you're using linode to host so it should be safe to send directly from the lemmy host. If you were hosting this on a residential connection you would need to setup an external SMTP relay (i.e. Amazon SES, Sendgrid, etc...) as most resi blocks are blacklisted.
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Create a valid DMARC record.
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Once the previous steps are fixed you need to petition Spamhaus to remove your IP from the RBL. You can cite having performed steps 1 through 3 to fix your issues if pressed.