why do emails for comments randomly fail?

I’m afraid I won’t be able to help you there, as these emails are not managed by the Jetpack plugin. They’re handled by WordPress itself, you do not need a plugin to manage such emails.

If your WordPress installation isn’t always able to send out emails, you could try to use a plugin like this, to bypass your server’s mail functions and use another email provider instead:
https://ww.wp.xz.cn/plugins/wp-mail-smtp/
https://ww.wp.xz.cn/plugins/postman-smtp/

Jeremy, why do you say the emails are not handled by Jetpack? […]
How is this not part of Jetpack?

I believe we’re talking about 2 different things here. Let me see if I can clarify, so you can let me know where you’re having trouble exactly.

I am still not getting notification emails when someone posts a comment on my Blog.

These notification emails are not handled by Jetpack. They’re part of WordPress itself. Whether you use a plugin or not, WordPress tries to send an email from your site to the post author every time a comment is posted, as long as that option is checked under Settings > Discussion in your dashboard.

If that doesn’t work on your site, it could be because your server cannot send out any emails, or because the emails are filtered out as spam before they reach your inbox. Something like Jason’s Postman plugin should help you figuring that out.

I am looking at the subscriptions part of the Jetpack plugin and I have “Notify me of follow up comments by email” and “Notify me of new posts by email” checked.

That’s indeed a Jetpack feature. However, it is meant to allow others to subscribe to your blog, or to specific posts on your site. It doesn’t work like WordPress’ default comment notification system. The emails are actually sent from WordPress.com servers, and are only sent to people who subscribe to the blog / post.

If you’re having trouble with that option, would you mind contacting me via this contact form, and let me know one of the posts where you experience issues, as well as the email address of one of the subscribers (it could be you, or one of your readers) who did not receive a notification email).

I’d recommend against posting that email address here, since the forums are public.

Thanks!

Just the domain! Domains are not, for the most part, secret. It’s hard to give advice about MX and SPF and whatnot without a domain to go on.

Agreed. Although I can’t do much about checking WordPress.com email logs with just a domain. 🙂 That recommendation applied to the second part of my post.