My therapist very rarely cancels. In 3 1/2 years I think she has had to cancel once due to illness. She is also well established, with 30 years of experience.
She works 4 days a week, leaving Friday free for long weekends to travel to the adjoining state where her daughter and family live, and for other travel she enjoys. She is independently employed so she sets her own hours. So she has room in her own schedule for things that come up.

I would not be happy to receive an email at work when I had not offered the email address to him. I wonder why he didn't call you at work. I would have had a strong reaction not only to the cancellation but to the unexpectedness and intrusiveness of the email. I hope you will let him know that emailing you crossed your boundary.