This might depend on the country's (or other political entity) laws and regulations, but it would be highly unethical for a professor to subvert the application of a student. Such things as student misconduct (if that is alleged), when important, are handled within the university and (one hopes) appropriate action taken there, and settled.

It would probably also be improper for the receiving university to consider it and the processes don't normally allow such things. But it is impossible to say what sorts of communication flows between colleagues at different institutions.

It would be, in my opinion and US perspective, grounds for tenure revocation or, at least, a reprimand. It would also take an especially vindictive person to do such a thing.

I think the risk is small.

I'll focus on "bad" letters and communications.

This might depend on the country's (or other political entity) laws and regulations, but it would be highly unethical for a professor to independently subvert the application of a student. Such things as student misconduct (if that is alleged), when important, are handled within the university and (one hopes) appropriate action taken there, and settled.

It would probably also be improper for the receiving university to consider it and the processes don't normally allow such things. But it is impossible to say what sorts of communication flows between colleagues at different institutions.

It would be, in my opinion and US perspective, grounds for tenure revocation or, at least, a reprimand. It would also take an especially vindictive person to do such a thing.

I think the risk of this happening is small.

This might depend on the country's (or other political entity) laws and regulations, but it would be highly unethical for a professor to subvert the application of a student. Such things, when important, are handled within the university and (one hopes) appropriate action taken there, and settled.

It would probably also be improper for the receiving university to consider it and the processes don't normally allow such things. But it is impossible to say what sorts of communication flows between colleagues at different institutions.

It would be, in my opinion and US perspective, grounds for tenure revocation or, at least, a reprimand. It would also take an especially vindictive person to do such a thing.

I think the risk is small.

This might depend on the country's (or other political entity) laws and regulations, but it would be highly unethical for a professor to subvert the application of a student. Such things as student misconduct (if that is alleged), when important, are handled within the university and (one hopes) appropriate action taken there, and settled.

It would probably also be improper for the receiving university to consider it and the processes don't normally allow such things. But it is impossible to say what sorts of communication flows between colleagues at different institutions.

It would be, in my opinion and US perspective, grounds for tenure revocation or, at least, a reprimand. It would also take an especially vindictive person to do such a thing.

I think the risk is small.