I think boundaries and policies in general always have to reflect the overall and current situation with an individual client. Blanket policies that apply to everyone equally seem to me to be more about the T's preferences/limitations than about supporting the therapy or the therapeutic relationship.

So in your e-mail example, if the second e-mail reflected something crucial, I think that generally would take precedence over the policy because of its impact on the therapy. I would see the T as doing the right thing to respond, and not see it as related to the on-going policy. I would suspect that it might lead to a revision of the policy, if the issue that prompted the e-mail will become a continuing concern.

But I think there's a difference between boundaries and the therapy frame though they interrelate. Boundaries usually concern isolated actions. But the frame can be affected by how boundaries are handled by both client and T. The establishment of and engaging with boundaries that don't support the therapeutic relationship and goals damage the frame, and so can undermine both the therapy and the relationship. Boundaries can come and go, but the frame needs to remain consistent.