There are few situations where customer would like to send an email to a mail address or reply to the issue via an email address, the emails are getting bounced when users are adding the custom domain mail handler instead of Default Cloud Mail Server in 

https://<site_name.atlassian.net/plugins/servlet/project-config/Project_Key/notifications 

Actual Results:

Emails are getting bounced when the users reply to the custom cloud mail server when they configured in notification scheme.

Expected Results:

Allow users to reply to the custom email server so that the comments are appended to the issue or create a new issue based on the configuration.