The account invite is a Shopify platform email so it’s just like other Shopify transactional emails for your store. The Shopify platform deploys the account invite emails to your customer when you use our app. Because the email is actually deployed by Shopify, the ISPs see the emails coming from a Shopify server, which in general sends a lot of emails to lots of customers for lots of stores, which could trigger some spam filter mechanisms at certain ISP. The ISPs look for email origination, email content, valid ‘from’ addresses, and more to determine whether an email is SPAM or not.
To ensure your emails make it to your customer’s inbox, it’s important your follow Shopifys guidelines for setting up email with your store here, including SPF records and DKIM: