Hi,
We are using Jira Cloud. Our customers have alerted us that comment creation by e-mail was not working. I tested it: I created a test organization, added my personal e-mail as "customer", created a ticket using our Service Desk portal.
I received a notification from Jira when the issue was created:

I tried replying to it twice: once I hit the "Reply" button which sent the reply to servicedesk@behavox.com, and one more time by replying to jira@behavox.atlassian.net.
We seem to have a vanilla configuration for Incoming Mail:
