[Nextgrid]

Fastmail is still like 0.1% of users. If you're a "growth & engagement" company you're probably better off banning that domain wholesale and as a bonus will get rid of tech-savvy ad/tracker blocker users without making any impact to your target market.

The advantage of iCloud is that it's a domain laymen use - those same laymen the "growth & engagement" scum wants to track and spam. They can't just ban it wholesale without alienating a large chunk of their target market (and a pretty lucrative one at that, since Apple hardware is expensive).