Use dedicated email sending IP addresses instead of shared IP pools for custom email domains sending

Issue Summary

This is a cloud specific suggestion. Custom email domains allows an administrator to have their Jira/JSM email notification sent from their own domain instead of their default Atlassian domain that looks like `jira@<tenant>.atlassian.net`. These emails are sent through AWS SES and use a shared IP pool.

We're seeing a couple of cases where the sending IP address is listed on a blocklist not due to Atlassian's sending behaviour, but because of another user of the SES shared IP pool. Atlassian using a shared pool is then exposed to transient deliverability issues as an IP address that SES chooses to send from may be on a blocklist that the recipient checks before receiving.

When an IP address is on a blocklist, it doesn't impact all of email deliverability as (a) a recipient's mailserver must choose to check that the IP is on a blocklist and if so, block it and (b) the SES shared IP pool has many IP addresses, with many not on the blocklist. So it's a transient email deliverability issue.

Suggested Solutions

Use dedicated IP addresses so Atlassian is solely in control of their IP reputation and blocklist events happen less frequently.

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available