Authentication & Identification
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.
Learn more about email authentication.
Additional guidelines for IPv6
The sending IP must have a PTR record (i.e., a reverse DNS of the sending IP) and it should match the IP obtained via the forward DNS resolution of the hostname specified in the PTR record. Otherwise, mail will be marked as spam or possibly rejected.
The sending domain should pass either SPF check or DKIM check. Otherwise, mail might be marked as spam.
Subscription
Each user on your distribution list should opt to receive messages from you in one of the following ways (opt-in):
Through an email asking to subscribe to your list.
By manually checking a box on a web form, or within a piece of software.
We also recommend that you verify each email address before subscribing them to your list.
The following methods of address collection are not considered 'opt-in' and are not recommended:
Using an email address list purchased from a third-party.
Setting a checkbox on a web form or within a piece of software to subscribe all users by default (requiring users to explicitly opt-out of mailings).