I offered to host a friends business email on my DO instance. Works 99% of the time but every now and then emails just disappear only to find out that MS and Apple block DO IP addresses, sometimes. Silently. There is a war on small email providers it seems.

The biggest war on small providers is waged by other small providers. They can be ancient and outdated or simply extremely picky. Which makes everything Google or others require a piece of cake, which it actually kinda is.