Hello @DavidS8 ,
Depending on the domain, there may have been an update through their IT / network that resulted in their email system rejecting external emails or emails coming through an ESP. With that said, here are some general best practices for delivery.
Delivery can have a lot of variables, from elements on our side, on your side, on the recipient's side, and in their email system company's side. I would strongly encourage you to swap to self-authentication since you have your own domain.
If you're wanting more in-depth, specialized insight on your current deliverability, it'd be worth speaking with one of our Delivery agents.
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