Occasionally, you or your users might send mail that bounces because the recipient blacklisted your IP address. When an IP address is blacklisted, all mail from the address gets blocked (bounces). Some domains use a blacklist service to identify and block mail from suspected spammers. If enough users mark mail received from a particular IP address as spam, the service can blacklist that address. Even if one of your messages goes out from an IP address that was blacklisted after someone else used it to send spam, your mail can bounce.
Option 1: Get added to approved senders
Ask the recipient to add your domain name to their approved senders list.
Option 2: Have recipient whitelist IP addresses
If the recipient’s mail service doesn't support approved senders, the recipient's IT admin should whitelist Gmail's sending IP addresses (listed below).
Whitelist these IP addresses
Ask the recipient’s domain IT administrator to add the following Gmail IP addresses to their SMTP gateway's whitelist:
What Google does to help
Google regularly asks blacklist services to remove our IP addresses if they get blacklisted. However, because we can't prevent services from blacklisting any of our IP addresses or guarantee the removal of them, Google doesn't remove blacklisted IP addresses.
Contact G Suite support
If you have additional questions or concerns, contact G Suite support.