My university blocks leaking/whistleblowing emails related to an internal investigation
This is quite an interesting question.  I may be wrong on this, but I do not think blocking those emails would constitute a breach of "academic freedom".  Academic freedom usually protects the freedom of inquiry and publication for academics in relation to their academic work, but it would not ordinarily extend to give them any special right of inquiry and receipt of information with respect to their employment conditions and the administration of their employment (beyond the normal expectations of workers in other fields).  From your description, the emails relate to the employment treatment of academics, rather than a subject of academic research/investigation, and so it would be likely that the university would have the same rights as any other employer in respect of these emails.  While it may be unseemly for the university to block communications to its staff that criticise the administration of their employment, it is unlikely that this would constitute a breach of the principles of academic freedom.

One simple solution that strikes me as obvious in this case would be for any interested recipients of these emails to reply to the sender and supply their personal email address, so that future correspondence can be sent to them at an email account that is not controlled by the university.  If the goal of the sender is simply to alert relevant employees of alleged misconduct by the university, then that can be accomplished by sending the email to their personal email accounts.