You need to remember that 1000 years ago, letters were not sent in envelopes (you folded the letter and sealed it), and postal services were limited to official government documents (if they existed at all). Private persons sent personal and business letters by finding someone they trusted who was going to the city where the addressee lived. An implied contract existed between the writer and the person delivering the letter was that they wouldn’t “peak”. So the takana in question (as applied to letters) was not so much a hidush (unlike the prohibition of polygamy) as a reinforcement of a standard contractual arrangement.
While I understand some “men” (if you call them that, as they probably aren’t mentches) dream of being able to afford a “harem” (and hope that the takana’s family law rules are no longer in effect), but any would be hackers looking for a heter should resign themselves that their hacking other people’s personal correspondence is glatt treff.