Ask HN: How bad is it to send Tax ID via plain text email?

I recently received an email from BrainTree Payments to verify my merchant information. They need to get the correct information for tax purposes, I get it.

I was dismayed to see my Tax ID and Legal Name next to each other in a plain text email.

I am a developer, but I do not have a lot of knowledge in the area of computer networking. So I am not 100% sure of the implications of sending sensitive information via plain text email.

How bad is this?