Alex here from Shopify's support team.
Good news - this is a pretty easy fix! What you'll need to do in this situation is add Shopify's SPF record to the TXT record in your custom domain settings.
The SPF record is
v=spf1 include:shops.shopify.com ~all
I hope this helps! There's some more information here, if you want to read up on it! Let me know if you have any other questions.
If you need to add other servers to your SPF, just add them to the same record in the following format...
v=spf1 include:shops.shopify.com include:_spf.google.com ~all
...where _spf.google.com has been added alongside the shops.shopify.com server.