Since you are getting them regularly as well as other customers, there is likely not an issue with the generation and sending of the emails.
It is most probably a problem along the send route or the customer's end.
First, have your customers check their Junk/Spam folders. Rules to detect these messages are constantly changing/evolving. Emails that once went through just fine, could suddenly begin going into these folders.
If the above is not the problem, some ISP's may also block senders (AppSheet is the actual sender) of what they consider are frequent "spam" messages. In these cases AppSheet would need to "re-validate" that their emails are legitimate. You would want to contact AppSheet so they can check if they are getting any of your emails returned for this reason and they can take the necessary action.
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