How to tell if frozen mussels are good?
'You shouldn't eat mussels that don't open' is an unfounded myth. It was first mentioned in a book by the British food writer Jane Grigson in the 1970s and grew through repetition to the point that 90% of cook books mentioned it in the 1990s.

The fact is, mussels that don't open after cooking are more likely to be safe than those that do.

You should, however, be careful to remove any mussels that have opened before cooking, as they'll be dead and likely to make you ill.