Backlog allows you to receive email notifications whenever issues are added/updated or commented on. At the same time, you can reply to the email notification to add a comment on the issue.
How to quickly comment on issues via email
When you receive an email notification about a Backlog issue in your email inbox, you can click Reply using your email client (e.g. Gmail or Outlook), and enter your comment in the email reply.
After sending the email, your reply will be added as a comment on the issue. 
This method of commenting on issues would be useful for quick replies, but please note that we are unable to notify/@mention other users in the comment.
To do that, you can click on Go to Issue in the email notification to open the issue in a new webpage and add your comment, and notify other users, as usual.
Tips:
- Your Backlog personal settings should be set to “receive emails” in order to get the email notification. See how to manage your email notifications.
- This function only works if you are replying to the notification using your registered email address in Backlog.
- File attachment via email is not supported. Attached files in the email will not be attached to the issue comment.
- You may not be able to register a comment via email even if the conditions above are met. It is dependent on some settings or the email software you are using.