Private messages are private between the sender and receiver.
Messages stay in your out box until the receiver decides to open them.
There is no compulsion on any member to open or reply to a PM if they choose not to.
Rules of Wrong Planet apply to the sending of PMs too - it is not permissible to send personal attacks, sexist, racist or homophobic attacks and so on.
Receivers are able to link any abusive PM for moderators to review in full in the original state, and action is taken as necessary.
Members are not always open to receiving a PM from a member they have not previously had cordial interaction with on the boards, and occasionally pseudo members join in order to send phishing PMs which are scams.
So it's up to the receiver, and finally, there is no point in sending a PM to a member who has not visited the site for years, as it is unlikely to ever be seen.