To ensure that Gmail can identify you:
Use a consistent IP address to send bulk mail.
Keep valid reverse DNS records for the IP address(es) from which you
send mail, pointing to your domain.
Use the same address in the 'From:' header on every bulk mail you send.
We also recommend the following:
Sign messages with DKIM. We do not authenticate messages signed with
keys using fewer than 1024 bits.
Publish an SPF record.
Publish a DMARC policy.
DKIM would be my recommendation.
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