Problem Definition

In Server, if you want to blocklist a spam email, then you have to mark it as spam from the mail server itself. However, in Cloud there is no access to the server itself to block email addresses because they are owned by Atlassian.

Suggestion

Allow JIRA admins to set emails as spam from JIRA Service Desk itself, by having a list of blocked email addresses.

Workaround

Use a different email address with your custom domain to control blocked email addresses from the server end.