I have training and expertise in Canadian privacy laws - I have actually facilitated seminars on the subject to senior management. Privacy laws in Canada play a big role in employment law. However, this situation is not so much a privacy issue as it is an employment issue.
Emails on company-owned computers and related equipment/software are private only to the owners of the company. In this situation, they are not private to OP, they are not private to Frank, they are not private to Paul.
Since Paul is an agent of the company and OP's boss, he has the legal authority to dictate, on the company's behalf, where OP's work emails are forwarded. Frank, however, does not.
OP needs to politely inform Frank that he cannot forward his Paul communications to Frank without Paul's permission. He then needs to go and tell Paul what Frank is asking him to do.
While it's certainly possible OP might get fired for this, it's not nearly as likely as it would be if this was an American situation. It's not only very expensive to fire people in Canada (no such thing as at-will employment law in Canada! mandated large severance packages abound!), in this situation it could well be illegal (this would depend largely on the content of the emails Frank wants to see). And unlike the US, wrongfully terminated employees don't need lawyers to seek redress - all they need to do is report the employer to the relevant enforcement agency and then a whole ton of expensive grief will fall on the employer's head. I've seen it, it can be bad. Sensible employers try to stay away from this kind of trouble.