Hello @omshantiwellnes thanks for your post. This could be a few different things. First, your customer could check their preferences here to see if they are opted in or out of marketing emails. It's possible they opted out at some point but maybe doesn't remember? You mentioned you are seeing they are still subscribed, but if they visit the link above, they could double check this.
You could try and send a "manual" email to their msn email address...do you have a store email account you can use to see if they get your "direct" email, rather than what is being sent through Square. Trying a process of elimination to see where/what the issue may be with receiving your emails.
I know this is often mentioned, but do you know if your customer has checked their spam email folder? That's something that often happens. Hope you can find a solution to this and maybe these ideas helped.
