Hyde here from Shopify.
I can understand why you'd want to resolve that as quickly as possible, after all, email contact with your customers is a crucial part of marketing.
Are you referring to Shopify emails? Or notifications? Either way, I think this community forum post here should provide you with some help in that regard. The idea is that you add an SPF record to your custom domain, which is super easy and should help, if not outright resolve, the issue for you:
1. Log in to your domain hosting account.
2. Find your TXT record, which usually appears beside the CNAME record and MX record.
3. Add Shopify's SPF record - `v=spf1 include:shops.shopify.com ~all` - to your domain's TXT record.
4. Save your changes.
There are also a few other suggestions that may help in this community forum post that I'd recommend you read through thoroughly too!
I hope that all helps!
All the best, Hyde.