It sounds formal enough to me that it doesn't seem entirely natural. Speaking from the point of view of British English.
You should have unsent it, rather than tell me you could've unsent it.
That would be the minimum alteration to seem entirely natural. Consider adding actually between 'should have' and 'unsent it', and just between 'rather than' and 'tell me'. There are other natural options. For instance,
You'd have been better to unsend it, rather than just tell me you could've.
Note one difference here is that the "unsend it" isn't repeated. Repeating it is fine, it's not ungrammatical and isn't necessarily even unnatural - it can serve to emphasise. However, it's also unnecessary, as it is implicit. It can be left out, or replaced with something like "do/done that". For example,
Why did you put the laundry on to tumble dry when I told you not to?
It would have been better to pretend to punch him, rather than actually do that.