Issue Summary

At the moment, Atlassian does not support the use of relay messages with third-party tools such as Proofpoint or Google SMTP relay. 

Atlassian's email services are designed to send notifications using the customer's domain, with DNS records configured on their server

It would be nice to have that supported since some customers require that configuration to use the custom email domains to send messages.

Workaround

At the moment there is no workaround for that apart from reaching out to the relevant support team of the third-party products for further assistance and explore alternative solutions that can align with Atlassian's supported configurations.