well, the communication isn't perfect.
A letter typed out & handed to the kids in addition to the email would have been better - yes, NigellasDealer's dd might easily have 'mislaid' it on the way home, which is why the email, or any other form of communication which doesn't depend on the child's reliability/honesty/memory is required.
However, the school did satisfy the statutory requirements: so I'd say an emailed reply acknowledging this but pointing out that a letter to back it up would be appreciated next time, might be the way to go.
The 'but what if I hadn't checked it & dd didn't tell me & I sent her to school?!' worry is needless. This does happen in schools. Sometimes communication fails, & sometimes parents have been known to simply decide they don't like the idea of their child being excluded & pack them off to school regardless.
The child would usually then be placed in internal exclusion/isolation until a parent could be contacted to collect, then probably be excluded on the Monday.
It's generally better to carry out exclusions immediately rather than postpone them (hence not originally delaying it till Monday to allow for a letter to be sent out) as it avoids a day where the child knows they have exclusion hanging over them, & is either distressed at the prospect or stomping around creating disruption...