Emails sent to Google mail addresses (both gmail.com and GSuite hosted domains) get temporary failures and get stuck in the Delivery queue.
421-4.7.0 This message does not have authentication information or fails to pass
421-4.7.0 authentication checks. To best protect our users from spam, the
421-4.7.0 message has been blocked. Please visit
421-4.7.0 https://support.google.com/mail/answer/81126#authentication for more
421 4.7.0 information.
Google has become very aggressive in limiting connections to senders to gmail.com addresses that do not have some form of sender authentication. Such as Sender Protection Framework (SPF), DKIM, and DMARC, configured in the sender's DNS records.
Per Google's Bulk Sender Guidelines, they recommend that DKIM signatures over 1024 bits, correct reverse DNS records, and DMARC be enabled.
Symantec recommends the addition of SPF, Sender ID, DKIM, and DMARC records to validate your mail delivery.
Symantec does offer Email Fraud Protection, a product which will automate the configuration and monitoring of sender authentication for SMG and various cloud and email services. For more information, please see Symantec Email Fraud Protection.
If self configuration is the best option, please visit Setting up sender authentication for outbound mail for suggestions on implementing SPF/Sender ID/DKIM/DMARC.